Semiconductor device

ABSTRACT

Defects in an oxide semiconductor film are reduced in a semiconductor device including the oxide semiconductor film. The electrical characteristics of a semiconductor device including an oxide semiconductor film are improved. The reliability of a semiconductor device including an oxide semiconductor film is improved. A semiconductor device including an oxide semiconductor layer; a metal oxide layer in contact with the oxide semiconductor layer, the metal oxide layer including an In-M oxide (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf); and a conductive layer in contact with the metal oxide layer, the conductive layer including copper, aluminum, gold, or silver is provided. In the semiconductor device, y/(x+y) is greater than or equal to 0.75 and less than 1 where the atomic ratio of In to M included in the metal oxide layer is In:M=x:y.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. One embodiment of thepresent invention particularly relates to a semiconductor device and amethod for manufacturing the semiconductor device.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. An electro-optical device, an image display device(also simply referred to as a display device), a semiconductor circuit,a light-emitting device, a power storage device, a memory device, and anelectronic appliance may include a semiconductor device.

2. Description of the Related Art

As semiconductor materials of transistors used for most display devicestypified by liquid crystal display devices and light-emitting displaydevices and most integrated circuits (ICs), silicon semiconductors suchas amorphous silicon, single crystal silicon, and polycrystallinesilicon are known. Furthermore, as other semiconductor materials, oxidesemiconductors have been attracting attention. For example, a techniquefor applying a transistor in which zinc oxide or In—Ga—Zn-based oxide isused as an oxide semiconductor for a channel, to a display device, isdisclosed (Patent Document 1). Furthermore, a technique to apply atransistor in which polycrystalline In—Ga oxide is used as an oxidesemiconductor for a channel, to a display device, is disclosed(Non-Patent Document 1).

In addition, to reduce wiring delay due to increase in wiring resistanceand parasitic capacitance caused by increase in size and definition of adisplay device, a technique to form a wiring using a low-resistancematerial such as copper, aluminum, gold, or silver is considered (PatentDocument 2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-096055-   [Patent Document 2] Japanese Published Patent Application No.    2004-133422

Non-Patent Document

-   [Non-Patent Document 1] Yasuhiro Terai et al., “A Polycrystalline    Oxide TFT Driven AM-OLED Display”, IDW'11, pp. 61-64

SUMMARY OF THE INVENTION

In a transistor including an oxide semiconductor, a large amount ofimpurity (typically, silicon, which is a constituent element of aninsulating layer; carbon, and copper, which is a constituent material ofa wiring) contained in an oxide semiconductor layer causes a reductionin electrical characteristics (e.g., on-state current characteristics)of the transistor.

Another object of one embodiment of the present invention is to reducethe concentration of impurity contained in an oxide semiconductor layerof a semiconductor device such as a transistor including an oxidesemiconductor. Another object of one embodiment of the present inventionis to improve electrical characteristics of a semiconductor device orthe like including an oxide semiconductor. Another object of oneembodiment of the present invention is to improve the reliability of asemiconductor device or the like including an oxide semiconductor.Another object of one embodiment of the present invention is to providea novel semiconductor device or the like.

Note that the description of these objects does not disturb thedescription of other objects. One embodiment of the present inventiondoes not necessarily achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor layer; a metal oxide layer in contactwith the oxide semiconductor layer, the metal oxide layer including anIn-M oxide (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf); and a conductivelayer in contact with the metal oxide layer, the conductive layerincluding copper, aluminum, gold, or silver. In the semiconductordevice, y/(x+y) is greater than or equal to 0.75 and less than 1 wherethe atomic ratio of In to M included in the metal oxide layer isIn:M=x:y.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer; a gate insulating layer in contactwith the gate electrode layer; an oxide semiconductor layer facing thegate electrode layer with the gate insulating layer positioned betweenthe gate electrode layer and the oxide semiconductor layer; a metaloxide layer in contact with the oxide semiconductor layer, the metaloxide layer including an In-M oxide (M is Ti, Ga, Y, Zr, La, Ce, Nd, orHf); and a pair of electrode layers in contact with the metal oxidelayer, the pair of electrode layers including copper, aluminum, gold, orsilver. In the semiconductor device, y/(x+y) is greater than or equal to0.75 and less than 1 where the atomic ratio of In to M included in themetal oxide layer is In:M=x:y.

Another embodiment of the present invention is a semiconductor deviceincluding a first gate electrode layer; a first gate insulating layer incontact with the first gate electrode layer; an oxide semiconductorlayer facing the first gate electrode layer with the first gateinsulating layer positioned between the first gate electrode layer andthe oxide semiconductor layer; a metal oxide layer in contact with theoxide semiconductor layer, the metal oxide layer including an In-M oxide(M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf); a pair of electrode layers incontact with the metal oxide layer, the pair of electrode layersincluding copper, aluminum, gold, or silver; a second gate insulatinglayer over and in contact with the pair of electrode layers, and asecond gate electrode layer facing the oxide semiconductor layer withthe second gate insulating layer positioned between the oxidesemiconductor layer and the second gate electrode layer. In thesemiconductor device, y/(x+y) is greater than or equal to 0.75 and lessthan 1 where the atomic ratio of In to M included in the metal oxidelayer is In:M=x:y, and the first gate electrode layer and the secondgate electrode layer are electrically connected to each other through anopening portion formed in the first gate insulating layer and the secondgate insulating layer.

In the semiconductor device, the oxide semiconductor layer may include afirst side surface and a second side surface in contact with the pair ofelectrodes, and a third side surface and a fourth side surface facingthe first gate electrode layer or the second gate electrode layer.

In the semiconductor device, gallium is preferably contained as theelement M.

In the semiconductor device, the oxide semiconductor layer may have astacked-layer structure including a first oxide semiconductor layer anda second oxide semiconductor layer between the first oxide semiconductorlayer and the metal oxide layer. In that case, the electron affinity ofthe second oxide semiconductor layer is preferably smaller than theelectron affinity of the first oxide semiconductor layer and preferablylarger than the electron affinity of the metal oxide layer.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10 and less thanor equal to 100, and accordingly also includes the case where the angleis greater than or equal to −5° and less than or equal to 50. The term“substantially parallel” indicates that the angle formed between twostraight lines is greater than or equal to −30° and less than or equalto 30°. The term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°. The term“substantially perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 60° and less than orequal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

According to one embodiment of the disclosed invention, theconcentration of impurities contained in the oxide semiconductor layercan be reduced. In accordance with one embodiment of the presentinvention, electrical characteristics of a semiconductor device or thelike including an oxide semiconductor can be improved. In accordancewith one embodiment of the present invention, the reliability of asemiconductor device or the like including an oxide semiconductor can beimproved. In accordance with one embodiment of the present invention, anovel semiconductor device or the like can be provided. Note that thedescription of these effects does not disturb the existence of othereffects. One embodiment of the present invention does not necessarilyachieve all the objects listed above. Other effects will be apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings;

FIGS. 1A and 1B are a plan view and a cross-sectional view of atransistor of one embodiment of the present invention;

FIGS. 2A and 2B are a plan view and a cross-sectional view of atransistor of one embodiment of the present invention;

FIGS. 3A and 3B are a plan view and a cross-sectional view of atransistor of one embodiment of the present invention;

FIGS. 4A and 4B are a plan view and a cross-sectional view of atransistor of one embodiment of the present invention;

FIGS. 5A to 5D are cross-sectional views illustrating a manufacturingprocess of a transistor of one embodiment of the present invention;

FIGS. 6A to 6C are cross-sectional views illustrating a manufacturingprocess of a transistor of one embodiment of the present invention;

FIGS. 7A to 7C are a plan view, a cross-sectional view, and a banddiagram of a transistor of one embodiment of the present invention;

FIGS. 8A to 8D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS;

FIGS. 9A and 9B show nanobeam electron diffraction patterns of oxidesemiconductor films and FIGS. 9C and 9D illustrate an example of atransmission electron diffraction measurement apparatus;

FIG. 10A shows an example of structural analysis by transmissionelectron diffraction measurement and FIGS. 10B and 10C show plan-viewTEM images;

FIGS. 11A to 11C show Id-Vg characteristics, a band diagram, and SIMSanalysis results of a transistor;

FIGS. 12A and 12B show Id-Vg characteristics and a band diagram of atransistor;

FIGS. 13A to 13C show Id-Vg characteristics, a band diagram, and SIMSanalysis results of a transistor;

FIGS. 14A to 14C show Id-Vg characteristics, a band diagram, and SIMSanalysis results of a transistor;

FIGS. 15A to 15C show Id-Vg characteristics, a band diagram, and SIMSanalysis results of a transistor;

FIG. 16 is a graph showing results of XRD measurement.

FIG. 17 is a band diagram;

FIGS. 18A to 18C are a block diagram and circuit diagrams illustrating aconfiguration of a display device of one embodiment of the presentinvention;

FIG. 19 illustrates a display module of one embodiment of the presentinvention;

FIGS. 20A to 20D are views illustrating electronic appliances accordingto embodiments of the present invention;

FIGS. 21A and 21B are a cross-sectional view and a band diagram of atransistor of one embodiment of the present invention;

FIGS. 22A to 22C are a plan view and cross-sectional views of atransistor of one embodiment of the present invention;

FIGS. 23A to 23D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS;

FIGS. 24A to 24C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD;

FIGS. 25A and 25B show electron diffraction patterns of a CAAC-OS;

FIG. 26 shows a change of crystal parts of an In—Ga—Zn oxide owing toelectron irradiation;

FIGS. 27A and 27B are schematic diagrams illustrating deposition modelsof a CAAC-OS and an nc-OS;

FIGS. 28A to 28C illustrate an InGaZnO₄ crystal and a pellet; and

FIGS. 29A to 29D are schematic diagrams illustrating a deposition modelof a CAAC-OS.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that the mode and details can be variouslychanged without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be interpreted asbeing limited to the description of Embodiments below. In addition, inthe following embodiments, the same portions or portions having similarfunctions are denoted by the same reference numerals or the samehatching patterns in different drawings, and description thereof willnot be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component may be exaggerated forclarity. Therefore, embodiments of the present invention are not limitedto such a scale.

In this specification and the like, ordinal numbers such as “first”,“second”, and the like are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.Therefore, for example, description can be made even when “first” isreplaced with “second” or “third”, as appropriate.

Functions of a “source” and a “drain” are sometimes interchanged witheach other as appropriate when the direction of current flow is changedin circuit operation, for example. Thus, in this specification and thelike, the terms “source” and “drain” can be replaced with each other.

Note that the term such as “over” or “below” in this specification andthe like does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode layer over a gate insulating layer” does not excludethe case where a component is placed between the gate insulating layerand the gate electrode layer. The same applies to the term “below”.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 50.The term “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°.

In this specification and the like, the trigonal and rhombohedralcrystal systems are included in the hexagonal crystal system.

Embodiment 1

In this embodiment, a semiconductor device that is one embodiment of thepresent invention and a method for manufacturing the semiconductordevice are described. Description is made with reference to FIGS. 1A and1B, FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5A to 5D,and FIGS. 6A to 6C.

<Structure Example 1 of Transistor>

FIGS. 1A to 1B are a plan view and a cross-sectional view of atransistor 200 included in a semiconductor device of this embodiment.The transistor 200 illustrated in FIGS. 1A and 1B is a channel-etchedtransistor. FIG. 1A is a plan view of the transistor 200, and FIG. 1B isa cross-sectional view taken along dashed dotted lines A1-A2 and B1-B2in FIG. 1A. Note that a substrate 100 and some components (e.g., a gateinsulating layer) of the transistor 200 are not illustrated in FIG. 1Afor simplicity.

The transistor 200 illustrated in FIGS. 1A and 1B includes a gateelectrode layer 102 formed over the substrate 100, a gate insulatinglayer 104 in contact with the gate electrode layer 102, an oxidesemiconductor layer 106 facing the gate electrode layer 102 with thegate insulating layer 104 positioned therebetween, a metal oxide layer108 over the oxide semiconductor layer 106, and a pair of electrodelayers 110 a and 110 b in contact with the metal oxide layer 108.Furthermore, the transistor 200 may include an oxide insulating layer112, an oxide insulating layer 114 and a nitride insulating layer 116formed over the pair of electrode layers 110 a and 110 b and the metaloxide layer 108.

In the transistor 200, the metal oxide layer 108, which is provided incontact with the top surface of the oxide semiconductor layer 106 wherea channel is formed, functions as a barrier layer for preventingdiffusion of constituent elements of the pair of electrode layers 110 aand 110 b into the oxide semiconductor layer 106. In addition, the metaloxide layer 108 can also prevent constituent elements of the oxideinsulating layer 112 or the like provided over the oxide semiconductorlayer 106 from mixing into the oxide semiconductor layer 106. Theprevention of mixing of impurities into the oxide semiconductor layer106 where the channel is formed can inhibit a reduction in theelectrical characteristics of the transistor 200.

For the metal oxide layer 108, a metal oxide represented as In-M oxide(M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) can be used. Note that toprevent the metal oxide layer 108 from functioning as part of a channelformation region, a material having sufficiently low conductivity isused. Alternatively, for the metal oxide layer 108, a material which hassmaller electron affinity (energy difference between the vacuum leveland the bottom of the conduction band) than the oxide semiconductorlayer 106 and has a difference in energy of the bottom of the conductionband from the oxide semiconductor layer 106 (i.e., has a band offset) isused. To prevent generation of a difference in threshold voltagedepending on the level of a drain voltage, the material of the metaloxide layer 108 is preferably selected so that the energy of the bottomof the conduction band of the metal oxide layer 108 is closer to thevacuum level than the energy of the bottom of the conduction band of theoxide semiconductor layer 106 by 0.2 eV or more, preferably 0.5 eV ormore.

In addition, increasing the atomic ratio of the element M to In canincrease the energy gap of the metal oxide layer 108 and reduce theelectron affinity. Accordingly, to prevent formation of a channel in themetal oxide layer 108 by forming the band offset of the conduction bandbetween the metal oxide layer 108 and the oxide semiconductor layer 106,y/(x+y) is preferably greater than or equal to 0.75 and less than 1,further preferably greater than or equal to 0.78 and less than 1, stillfurther preferably greater than or equal to 0.80 and less than 1 wherethe atomic ratio of In to M included in the metal oxide layer isIn:M=x:y. Note that, an element that is not indium, M, nor oxygen, whichare main components of the metal oxide layer 108, may be mixed to themetal oxide layer 108 as an impurity. The concentration of the impurityin this case is preferably less than or equal to 0.1%. The atomic ratioof In:M=x:y can be measured by inductively coupled plasma massspectrometry (ICP-MS). In:M=x:y refers to not the composition of thetarget but the composition of the metal oxide film obtained by asputtering method, and y/(x+y) is greater than or equal to 0.75 and lessthan 1 where the atomic ratio is In:M=x:y.

In the case where the metal oxide layer 108 is formed by a sputteringmethod, when the atomic ratio of the element M to In is increased, thenumber of particles in deposition can be reduced. To reduce the numberof particles, y/(x+y) may be greater than or equal to 0.90, e.g., 0.93where the atomic ratio is In:M=x:y. Note that in the case where themetal oxide layer 108 is formed by a sputtering method, when the atomicratio of M to In is too high, the insulating property of a targetbecomes high, which makes it difficult to perform deposition using DCdischarge; as a result, it is necessary to use RF discharge.Accordingly, when deposition is performed using DC discharge, which isapplicable to the case of using a large-sized substrate, y/(x+y) is setless than or equal to 0.96, preferably less than or equal to 0.95, e.g.,0.93. The use of the deposition method applicable to the case of using alarge-sized substrate can increase the productivity of the semiconductordevice.

Note that in the transistor 200, side surfaces of the oxidesemiconductor layer 106 where the channel is formed are in contact withthe pair of electrode layers 110 a and 110 b functioning as a sourceelectrode layer and a drain electrode layer, and in the contact regions,a source region and a drain region are formed. Therefore, the metaloxide layer 108 may have an insulating property.

Note that it is preferable that the metal oxide layer 108 not have aspinel crystal structure. This is because if the metal oxide layer 108has a spinel crystal structure, a constituent element of the pair ofelectrode layers 110 a and 110 b might be diffused into the oxidesemiconductor layer 106 owing to the spinel crystal structure. Forexample, it is preferable that an In-M oxide be used as the metal oxidelayer 108 and that a divalent metal element (e.g., zinc) not becontained as M, in which case the formed metal oxide layer 108 does nothave a spinel crystal structure.

The thickness of the metal oxide layer 108 is greater than or equal to athickness that is capable of inhibiting diffusion of the constituentelement of the pair of electrode layers 110 a and 110 b into the oxidesemiconductor layer 106, and less than a thickness which inhibits supplyof oxygen from the oxide insulating layer 112 to the oxide semiconductorlayer 106. For example, when the thickness of the metal oxide layer 108is greater than or equal to 10 nm, the constituent element of the pairof electrode layers 110 a and 110 b can be prevented from diffusing intothe oxide semiconductor layer 106. When the thickness of the metal oxidelayer 108 is less than or equal to 100 nm, oxygen can be effectivelysupplied from the oxide insulating layers 112 and 114 to the oxidesemiconductor layer 106.

In the transistor 200 described in this embodiment, the pair ofelectrode layers 110 a and 110 b functioning as source and drainelectrode layers are preferably formed with a single layer or a stackedlayer of a single metal that is a low-resistance material, such ascopper, aluminum, gold, or silver; an alloy containing any of thesematerials, or a compound containing any of these materials as a maincomponent. The pair of electrode layers 110 a and 110 b also functionsas wirings; therefore, even in the case where a large-sized substrate isused as the substrate 100, when the electrode layers are formed tocontain a low-resistance material such as copper, aluminum, gold, orsilver, a semiconductor device in which wiring delay is suppressed canbe manufactured.

In the case where the pair of electrode layers 110 a and 110 b has atwo-layer structure, the pair of electrode layers 110 a and 110 b isformed so that the second conductive layer is thick and contains asingle metal that is a low-resistance material, such as copper,aluminum, gold, or silver, an alloy containing any of these materials,or a compound containing any of these components as a main component;and a conductor functioning as a barrier layer against a conductor ofthe second conductive layer is used for the first conductive layer thatis in contact with the side surface of the oxide semiconductor layer 106and the side surface and top surface of the metal oxide layer 108. Forexample, a conductive layer of titanium, tantalum, molybdenum, tungsten;an alloy containing any of these elements; or a conductive layercontaining titanium nitride, tantalum nitride, molybdenum nitride,tungsten nitride; or the like can be used as the barrier layer. In thecase where the pair of electrode layers 110 a and 110 b has athree-layer structure, the third conductive layer is preferably formedusing a conductor functioning as a barrier layer against a conductor ofthe second conductive layer so as to be over and in contact with thefirst and second conductive layers.

In the case where the pair of electrode layers 110 a and 110 b has atwo-layer structure, for example, any of the following structures ispreferably used; a structure in which an aluminum film is stacked on atitanium film; a structure in which a copper film is stacked on atungsten film; a structure in which an aluminum film is stacked on atungsten film; a structure in which a copper film is stacked on acopper-magnesium-aluminum alloy film; a structure in which a copper filmis stacked on a titanium film; and a structure in which a copper film isstacked on a tungsten film. In the case where the pair of electrodelayers 110 a and 110 b has three-layer structure, a film formed oftitanium, titanium nitride, molybdenum, or molybdenum nitride ispreferably formed as each of the first and third conductive layers, anda film formed of a low-resistance material such as copper, aluminum,gold, or silver is preferably formed as the second conductive layer.

The pair of electrode layers functioning as source and drain electrodelayers in the transistor 200 described in this embodiment is formedusing electrode layers including a low-resistance material such ascopper, aluminum, gold, or silver, whereby the semiconductor device inwhich wiring delay is suppressed can be manufactured. Furthermore, themetal oxide layer 108 functioning as a barrier layer is provided incontact with the pair of electrode layers, whereby a reduction inelectrical characteristics can be prevented, and thus it is possible toprovide a semiconductor device having favorable electricalcharacteristics.

Note that the number of masks may be reduced by forming the electrodelayers 110 a and 110 b, the oxide semiconductor layer 106, and the metaloxide layer 108 with the use of a half-tone mask (or a gray-tone mask, aphase difference mask, or the like), so that the number of processingsteps may be reduced. In this case, a pattern is formed by, for example,ashing of a resist. Therefore, the oxide semiconductor layer 106 and themetal oxide layer 108 are necessarily provided below the electrodelayers 110 a and 110 b. FIGS. 22A to 22C are a plan view andcross-sectional views of the structure in FIGS. 1A and 1B in the casewhere a half-tone mask is used.

Other constituent elements of the semiconductor device of thisembodiment are described below in detail.

(Substrate)

There is no particular limitation on a material or the like of thesubstrate 100 as long as the material has heat resistance enough towithstand at least heat treatment to be performed later. For example, aglass substrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 100. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOI(silicon on insulator) substrate, or the like may be used as thesubstrate 400. Furthermore, any of these substrates further providedwith a semiconductor element may be used as the substrate 100. In thecase where a glass substrate is used as the substrate 100, a glasssubstrate having any of the following sizes can be used: the 6thgeneration (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm),and the 10th generation (2950 mm×3400 mm). Thus, a large-sized displaydevice can be manufactured.

Further alternatively, a flexible substrate may be used as the substrate100, and the transistor 200 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 100 and the transistor 200. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate 100 and transferredonto another substrate. In that case, the transistor 200 can betransferred to a substrate having low heat resistance or a flexiblesubstrate.

(Gate Electrode Layer)

The gate electrode layer 102 can be formed using a metal elementselected from chromium, copper, aluminum, gold, silver, zinc,molybdenum, tantalum, titanium, and tungsten; an alloy containing any ofthese metal elements as a component; an alloy containing any of thesemetal elements in combination; or the like. Further, one or more metalelements selected from manganese and zirconium may be used. Furthermore,the gate electrode layer 102 may have a single-layer structure or astacked-layer structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, an alloy film or a nitride film whichcontains aluminum and one or more elements selected from titanium,tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may beused.

The gate electrode layer 102 can also be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is also possible to have astacked-layer structure formed using the above light-transmittingconductive material and the above metal element.

Further, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-basedoxynitride semiconductor film, an In—Ga-based oxynitride semiconductorfilm, an In—Zn-based oxynitride semiconductor film, a Sn-basedoxynitride semiconductor film, an In-based oxynitride semiconductorfilm, a film of metal nitride (such as InN or ZnN), or the like may beprovided between the gate electrode layer 102 and the gate insulatinglayer 104. These films each have a work function of 5 eV or higher,preferably 5.5 eV or higher, which is higher than the electron affinityof an oxide semiconductor; thus, the threshold voltage of a transistorincluding the oxide semiconductor can be shifted in the positivedirection. Accordingly, a switching element having what is callednormally-off characteristics is obtained. For example, in the case ofusing an In—Ga—Zn-based oxynitride semiconductor film, an In—Ga—Zn-basedoxynitride semiconductor film having a higher nitrogen concentrationthan at least the oxide semiconductor layer 106, specifically, anIn—Ga—Zn-based oxynitride semiconductor film having a nitrogenconcentration of 7 atomic % or higher is used.

(Gate Insulating Layer)

The gate insulating layer 104 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, hafnium oxide, gallium oxide, and Ga—Zn-basedmetal oxide.

Alternatively, the gate insulating layer 104 may be formed using ahigh-k material such as hafnium silicate (HfSiO_(x)), hafnium silicateto which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate towhich nitrogen is added (HfAl₃O_(y)N_(z)), hafnium oxide, or yttriumoxide, in which case gate leakage current of the transistor can bereduced.

The thickness of the gate insulating layer 104 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

(Oxide Semiconductor Layer)

The oxide semiconductor layer 106 is typically formed using an In—Gaoxide, an In—Zn oxide, or an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La,Ce, Nd, or Hf).

In the case where the oxide semiconductor layer 106 is an In-M-Zn oxide(M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), it is preferable that theatomic ratio of metal elements of a sputtering target used for forming afilm of the In-M-Zn oxide satisfy In≧M and Zn≧M. As the atomic ratio ofmetal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that the atomicratio of metal elements in the formed oxide semiconductor layer 106varies from the above atomic ratio of metal elements of the sputteringtarget within a range of ±40% as an error.

In the case of using an In-M-Zn oxide for the oxide semiconductor layer106, when Zn and O are eliminated from consideration, the atomicpercentage of In and the atomic percentage of M are preferably greaterthan 25 atomic % and less than 75 atomic %, respectively, furtherpreferably greater than 34 atomic % and less than 66 atomic %,respectively.

The energy gap of the oxide semiconductor layer 106 is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. With the useof an oxide semiconductor having such a wide energy gap, the off-statecurrent of the transistor 200 can be reduced.

The thickness of the oxide semiconductor layer 106 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

An oxide semiconductor layer with low carrier density is used as theoxide semiconductor layer 106. For example, an oxide semiconductor layerwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, still further preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor layer 106.

Note that, without limitation to the compositions and materialsdescribed above, a material with an appropriate composition may be useddepending on required semiconductor characteristics and electricalcharacteristics (e.g., field-effect mobility and threshold voltage) of atransistor. Furthermore, in order to obtain the required semiconductorcharacteristics of the transistor, it is preferable that the carrierdensity, the impurity concentration, the defect density, the atomicratio of a metal element to oxygen, the interatomic distance, thedensity, and the like of the oxide semiconductor layer 106 be set toappropriate values.

Note that it is preferable to use, as the oxide semiconductor layer 106,an oxide semiconductor layer in which the impurity concentration is lowand the density of defect states is low, in which case the transistorcan have more excellent electrical characteristics. Here, the state inwhich impurity concentration is low and density of defect states is low(the number of oxygen vacancies is small) is referred to as “highlypurified intrinsic” or “highly purified substantially intrinsic”. Ahighly purified intrinsic or highly purified substantially intrinsicoxide semiconductor has few carrier generation sources, and thus canhave a low carrier density. Thus, a transistor including the oxidesemiconductor layer in which a channel region is formed rarely has anegative threshold voltage (is rarely normally-on). Thus, the transistorincluding the oxide semiconductor layer in the channel formation regionhas a small variation in electrical characteristics and high reliabilityin some cases. Furthermore, the highly purified intrinsic or highlypurified substantially intrinsic oxide semiconductor layer has anextremely low off-state current; even when an element has a channelwidth of 1×10⁶ μm and a channel length L of 10 μm, the off-state currentcan be less than or equal to the measurement limit of a semiconductorparameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage(drain voltage) between a source electrode and a drain electrode in therange from 1 V to 10 V.

Thus, the transistor in which the channel region is formed in the highlypurified intrinsic or highly purified substantially intrinsic oxidesemiconductor layer can have a small variation in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor in which the channelregion is formed in the oxide semiconductor layer having a high densityof defect states may have unstable electrical characteristics. Asexamples of the impurities, hydrogen, nitrogen, alkali metal, alkalineearth metal, and the like are given.

Hydrogen contained in the oxide semiconductor layer reacts with oxygenbonded to a metal atom to be water, and also causes oxygen vacancy in alattice from which oxygen is released (or a portion from which oxygen isreleased). Due to entry of hydrogen into the oxygen vacancy, an electronserving as a carrier is generated. Furthermore, in some cases, bondingof part of hydrogen to oxygen bonded to a metal element causesgeneration of an electron serving as a carrier. Thus, a transistorincluding an oxide semiconductor which contains hydrogen is likely to benormally on. Accordingly, it is preferable that hydrogen be reduced asmuch as possible in the oxide semiconductor layer 106. Specifically, inthe oxide semiconductor layer 106, the concentration of hydrogen whichis measured by secondary ion mass spectrometry (SIMS) is lower than orequal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³,or lower than 5×10¹⁸ atoms/cm³, preferably lower than or equal to 1×10¹⁸atoms/cm³, further preferably lower than or equal to 5×10¹⁷ atoms/cm³,still further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor layer 106, the amount of oxygenvacancy is increased, and the oxide semiconductor layer 106 is changedto an n-type. Thus, the concentration of silicon or carbon (theconcentration is measured by SIMS) in the oxide semiconductor layer 106or the concentration of silicon or carbon (the concentration is measuredby SIMS) in the vicinity of the interface between the metal oxide layer108 and the oxide semiconductor layer 106 is set to be lower than orequal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷atoms/cm³.

Furthermore, the concentration of alkali metal or alkaline earth metalof the oxide semiconductor layer 106, which is measured by SIMS, islower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal mightgenerate carriers when bonded to an oxide semiconductor, in which casethe off-state current of the transistor might be increased. Therefore,it is preferable to reduce the concentration of alkali metal or alkalineearth metal of the oxide semiconductor layer 106.

In addition, when nitrogen is contained in the oxide semiconductor layer106, electrons serving as carriers are generated to increase the carrierdensity, so that the oxide semiconductor layer 106 easily becomesn-type. Thus, a transistor including an oxide semiconductor whichcontains nitrogen is likely to be normally on. For this reason, nitrogenin the oxide semiconductor film is preferably reduced as much aspossible; the concentration of nitrogen which is measured by SIMS ispreferably set to, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor layer 106 may have a non-single crystalstructure, for example. The non-single crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure described later, or an amorphous structure, for example. Amongthe non-single crystal structure, the amorphous structure has thehighest density of defect levels, whereas CAAC-OS has the lowest densityof defect levels.

The oxide semiconductor layer 106 may have an amorphous structure, forexample. The oxide semiconductor film having the amorphous structure hasdisordered atomic arrangement and no crystalline component, for example.Alternatively, the oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor layer 106 may be a mixed filmincluding two or more of the following; a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a region of CAAC-OS described later, and aregion having a single-crystal structure. The mixed film includes, forexample, two or more of a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a CAAC-OS region, and a region having a single-crystalstructure in some cases. Further, the mixed film has a stacked-layerstructure of two or more of a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Note that the metal oxide layer108 in contact with the oxide semiconductor layer 106 can have anamorphous structure, a microcrystalline structure, a polycrystallinestructure, or the like, for example.

In the case where an insulating layer which contains a differentconstituent element (e.g., silicon) from the oxide semiconductor isprovided in contact with the oxide semiconductor layer 106, an interfacestate due to heterojunction, entry of impurities, or the like might beformed at the interface between the oxide semiconductor layer 106 andthe insulating layer. In the transistor 200 of this embodiment, themetal oxide layer 108 which has the same constituent element as theoxide semiconductor is provided between the oxide semiconductor layer106 and the oxide insulating layer 112 which may have a differentconstituent element (e.g., silicon) from the oxide semiconductor. Hence,if trap states are formed between the metal oxide layer 108 and theoxide insulating layer 112 owing to impurities and defects, electronsflowing in the oxide semiconductor layer 106 are less likely to becaptured by the trap states because there is a distance between the trapstates and the oxide semiconductor layer 106. Accordingly, the amount ofon-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are capturedby the trap states, the electrons become negative fixed charges. As aresult, the threshold voltage of the transistor fluctuates. However, bythe distance between the oxide semiconductor layer 106 and the trapstates, capture of the electrons by the trap states can be reduced, andaccordingly a fluctuation of the threshold voltage can be reduced.

The element M contained in the metal oxide layer 108 has a high bondingstrength to oxygen; therefore, oxygen vacancy is less likely to formedin the metal oxide layer 108 in which the atomic ratio of the element Mis high. Therefore, it is possible to reduce the amount of oxygenvacancy in the oxide semiconductor layer 106 in contact with the metaloxide layer 108.

(Oxide Insulating Layer)

The oxide insulating layer 112 is an oxide insulating film through whichoxygen is passed. Note that the oxide insulating layer 112 alsofunctions as a film which relieves damage to the metal oxide layer 108and the oxide semiconductor layer 106 when the oxide insulating layer114 formed later is formed.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the oxide insulating layer 112. Note that in thisspecification, “silicon oxynitride film” refers to a film that containsmore oxygen than nitrogen, and “silicon nitride oxide film” refers to afilm that contains more nitrogen than oxygen.

In addition, it is preferable that the number of defects in the oxideinsulating layer 112 be small and typically, the spin density of asignal that appears at g=2.001 due to a dangling bond of silicon belower than or equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR)measurement. This is because if the density of defects in the oxideinsulating layer 112 is high, oxygen is bonded to the defects and theamount of oxygen that passes through the oxide insulating layer 112 isdecreased.

Moreover, it is preferable that the amount of defects at the interfacebetween the oxide insulating layer 112 and the metal oxide layer 108 besmall, typically the spin density corresponding to a signal whichappears at g of greater than or equal to 1.89 and less than or equal to1.93 due to an oxygen vacancy in the metal oxide layer 108 be lower thanor equal to 1×10¹⁷ spins/cm³, more preferably lower than or equal to thelower limit of detection by ESR measurement.

Note that all oxygen entering the oxide insulating layer 112 from theoutside does not move to the outside of the oxide insulating layer 112and some oxygen remains in the oxide insulating layer 112. Furthermore,movement of oxygen occurs in the oxide insulating layer 112 in somecases in such a manner that oxygen enters the oxide insulating layer 112and oxygen contained in the oxide insulating layer 112 moves to theoutside of the oxide insulating layer 112. When an oxide insulating filmwhich is permeable to oxygen is formed as the oxide insulating layer112, oxygen released from the oxide insulating layer 114 provided overthe oxide insulating layer 112 can be moved to the oxide semiconductorlayer 106 through the oxide insulating layer 112.

The oxide insulating layer 114 is formed in contact with the oxideinsulating layer 112. The oxide insulating layer 114 is formed using anoxide insulating film whose oxygen content is in excess of that in thestoichiometric composition. Part of oxygen is released by heating fromthe oxide insulating film containing more oxygen than that in thestoichiometric composition. The oxide insulating film containing moreoxygen than that in the stoichiometric composition is an oxideinsulating film of which the amount of released oxygen converted intooxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, preferablygreater than or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis. Note thatthe substrate temperature in the TDS analysis is preferably higher thanor equal to 100° C. and lower than or equal to 700° C., or higher thanor equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used for the oxide insulating layer 114.

It is preferable that the amount of defects in the oxide insulatinglayer 114 be small, and typically the spin density corresponding to asignal which appears at g=2.001 due to a dangling bond of silicon, belower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxide insulatinglayer 114 is provided more apart from the oxide semiconductor layer 106than the oxide insulating layer 112 is; thus, the oxide insulating layer114 may have higher defect density than the oxide insulating layer 112.

(Nitride Insulating Layer)

It is possible to prevent outward diffusion of oxygen from the oxidesemiconductor layer 106 and entry of hydrogen, water, and the like intothe oxide semiconductor layer 106 from the outside by providing thenitride insulating layer 116 having a blocking effect against oxygen,hydrogen, water, alkali metal, alkaline earth metal, and the like overthe oxide insulating layer 114. The nitride insulating layer is formedusing silicon nitride, silicon nitride oxide, aluminum nitride, aluminumnitride oxide, or the like. Note that instead of the nitride insulatinglayer having a blocking effect against oxygen, hydrogen, water, alkalimetal, alkaline earth metal, and the like, an oxide insulating layerhaving a blocking effect against oxygen, hydrogen, water, and the like,may be provided. As the oxide insulating layer having a blocking effectagainst oxygen, hydrogen, water, and the like, aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, and hafnium oxynitride can be given.

<Structure Example 2 of Transistor>

FIGS. 2A to 2B are a plan view and a cross-sectional view of atransistor 210 included in a semiconductor device of this embodiment.FIG. 2A is a plan view of the transistor 210, and FIG. 2B is across-sectional view taken along dashed dotted lines A3-A4 and B3-B4 inFIG. 2A. Note that the substrate 100 and some components (e.g., a gateinsulating layer) of the transistor 210 are not illustrated in FIG. 2Afor simplicity.

The transistor 210 illustrated in FIGS. 2A and 2B includes the gateelectrode layer 102 formed over the substrate 100; the gate insulatinglayer 104 in contact with the gate electrode layer 102; an oxidesemiconductor layer 206 facing the gate electrode layer 102 with thegate insulating layer 104 positioned therebetween; the metal oxide layer108 over the oxide semiconductor layer 206; the pair of electrode layers110 a and 110 b in contact with the metal oxide layer 108; the oxideinsulating layer 112, the oxide insulating layer 114, and the nitrideinsulating layer 116 formed over the pair of electrode layers 110 a and110 b and the metal oxide layer 108; and an electrode layer 118 formedover the nitride insulating layer 116.

The electrode layer 118 functions as a back gate electrode in thetransistor 210. A stacked-layer structure that includes the oxideinsulating layer 112, the oxide insulating layer 114, and the nitrideinsulating layer 116 and is provided between the electrode layer 118 andthe oxide semiconductor layer 206 functions as a gate insulating layerfor the back gate electrode. The electrode layer 118 is connected to thegate electrode layer 102 through opening portions 117 a and 117 b formedin the gate insulating layer 104, the oxide insulating layer 112, theoxide insulating layer 114, and the nitride insulating layer 116.Therefore, the same potential is applied to the electrode layer 118 andthe gate electrode layer 102.

The transistor 210 in FIGS. 2A and 2B is different from the transistor200 in FIGS. 1A and 1B in that the electrode layer 118 functioning as aback gate electrode is provided over the nitride insulating layer 116.The other structures are the same as those of the transistor 200 and theeffect similar to that in the case of the transistor 200 can beobtained. That is, the transistor 210 includes the metal oxide layer 108which functions as a barrier layer and which is positioned between thepair of electrode layers 110 a and 110 b containing a low-resistancematerial and the oxide semiconductor layer 206 where a channel isformed. Thus, entry and diffusion of impurities to the oxidesemiconductor layer 206 can be prevented. Thus, a reduction in theelectrical characteristics is inhibited in the transistor 210. Fordetails of every component in the transistor 210, the description of thetransistor 200 can be referred to.

The oxide semiconductor layer 206 included in the transistor 210 inFIGS. 2A and 2B is formed using the same material as the oxidesemiconductor layer 106 included in the transistor 200, and has athickness greater than or equal to 100 nm, for example, greater than orequal to 100 nm and less than or equal to 1000 nm, preferably greaterthan or equal to 200 nm and less than or equal to 1000 nm. The channellength of the transistor 210 (the distance between the pair of electrodelayers 110 a and 110 b) is preferably greater than or equal to 0.5 μmand less than or equal to 2 μm, further preferably greater than or equalto 0.5 μm and less than or equal to 1 μm.

As illustrated in the cross-sectional view of FIG. 2B, the oxidesemiconductor layer 206 faces each of the gate electrode layer 102 andthe electrode layer 118 (back gate electrode) to be positioned betweenthe two electrode layers. The lengths in the channel length directionand the channel width direction of the electrode layer 118 functioningas a back gate electrode are longer than those of the oxidesemiconductor layer 206, respectively. The whole oxide semiconductorlayer 206 is covered with the electrode layer 118 with the insulatinglayers (the oxide insulating layer 112, the oxide insulating layer 114,and the nitride insulating layer 116) positioned therebetween.Furthermore, since the electrode layer 118 and the gate electrode layer102 are connected to each other through the opening portions 117 a and117 b formed in the gate insulating layer 104, the oxide insulatinglayer 112, the oxide insulating layer 114, and the nitride insulatinglayer 116, side surfaces of the oxide semiconductor layer 206 in thechannel width direction face the back gate electrode (electrode layer118) with the insulating layers (the oxide insulating layer 112, theoxide insulating layer 114, and the nitride insulating layer 116)positioned therebetween.

Such a structure enables electric fields of the gate electrode layer 102and the electrode layer 118 to electrically surround the oxidesemiconductor layer 206 included in the transistor 210. A devicestructure of a transistor, like that of the transistor 210, in whichelectric fields of a gate electrode layer and a back gate electrodeelectrically surround an oxide semiconductor layer where a channel isformed can be referred to as a surrounded channel (s-channel) structure.

Since the transistor 210 has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor layer 206 by the gate electrode layer 102; therefore, thecurrent drive capability of the transistor 210 can be improved and highon-state current characteristics can be obtained. Since the on-statecurrent can be increased, it is possible to reduce the size of thetransistor 210. Furthermore, since the transistor 210 has a structure inwhich the channel is surrounded by the gate electrode layer 102 and theelectrode layer 118, the mechanical strength of the transistor 210 canbe increased.

Note that in the structure of the transistor 210, any one of the openingportions 117 a and 117 b may be formed, and the electrode layer 118 andthe gate electrode layer 102 may be connected to each other through theopening portion.

Note that the pair of electrode layers 110 a and 110 b included in thetransistor 210 has stacked-layer structures including first conductivelayers 109 a and 109 b and second conductive layers 111 a and 111 b. Anyof the materials given in the description of the first layer of theelectrode layers 110 a and 110 b can be used for the first conductivelayers 109 a and 109 b, as appropriate. In addition, any of thematerials given in the description of the second layer of the electrodelayers 110 a and 110 b can be used for the second conductive layers 111a and 111 b, as appropriate. Note that the structure of the pair ofelectrode layers 110 a and 110 b of the transistor 210 is not limited tothat illustrated in FIGS. 2A and 2B as long as the pair of the electrodelayers 110 a and 110 b contain copper, aluminum, gold, or silver, andmay be a single-layer structure or a stacked-layer structure of threelayers or more.

<Structure Example 3 of Transistor>

FIGS. 3A to 3B are a plan view and a cross-sectional view of atransistor 220 included in a semiconductor device of this embodiment.The transistor 220 is a modification example of the transistor 210 inFIGS. 2A and 2B. FIG. 3A is a plan view of the transistor 220, and FIG.3B is a cross-sectional view taken along dashed dotted lines A5-A6 andB5-B6 in FIG. 3A. Note that the substrate 100 and some components (e.g.,a gate insulating layer) of the transistor 220 are not illustrated inFIG. 3A for simplicity.

The transistor 220 illustrated in FIGS. 3A and 3B includes the gateelectrode layer 102 formed over the substrate 100; the gate insulatinglayer 104 in contact with the gate electrode layer 102; the oxidesemiconductor layer 206 facing the gate electrode layer 102 with thegate insulating layer 104 positioned therebetween; the metal oxide layer108 functioning as a barrier layer and provided over the oxidesemiconductor layer 206; the pair of electrode layers 110 a and 110 b incontact with the metal oxide layer 108; the oxide insulating layer 112,the oxide insulating layer 114, and the nitride insulating layer 116formed over the pair of electrode layers 110 a and 110 b and the metaloxide layer 108; and electrode layers 119 a, 119 b, and 119 c formedover the nitride insulating layer 116.

In the transistor 220, the electrode layer 119 b, which overlaps theoxide semiconductor layer 206 with the metal oxide layer 108 and theinsulating layers (the oxide insulating layer 112, the oxide insulatinglayer 114, and the nitride insulating layer 116) positionedtherebetween, functions as a back gate electrode. The electrode layers119 a and 119 c, which are formed in the same layer as the electrodelayer 119 b, are connected to the gate electrode layer 102, through theopening portions 117 a and 117 b, respectively, which are formed in thegate insulating layer 104, the oxide insulating layer 112, the oxideinsulating layer 114, and the nitride insulating layer 116. That is, theelectrode layers 119 a and 119 c function as part of the gate electrodelayer 102.

The transistor 220 is different from the transistor 210 in that theelectrode layer 118, which functions as a back gate electrode in thetransistor 210, is separated. The other components of the transistor 220can be similar to those of the transistor 210. The description of thetransistor 210 can be referred to for details of the structure of thetransistor 220.

The electrode layers 119 a and 119 c included in the transistor 220 haveregions which overlap the oxide semiconductor layer 206 when seen fromthe above, and face the side surfaces of the oxide semiconductor layer206 in the opening portions 117 a and 117 b. Thus, like the transistor210, the transistor 220 also has an s-channel structure in which theoxide semiconductor layer 206 is electrically surrounded by the gateelectrode layer 102 and the electrode layers 119 a, 119 b, and 119 c;therefore, an electric field for inducing a channel can be effectivelyapplied to the oxide semiconductor layer 206 by the gate electrode layer102. Accordingly, the current drive capability of the transistor 220 isincreased, so that high on-state current can be obtained.

Furthermore, since the electrode layer 119 b functioning as a back gateelectrode is not electrically connected to the gate electrode layer 102in the transistor 220, different potentials or signals can be input tothe gate electrode layer 102 and the electrode layer 119 b. Therefore,by a signal or potential input to the electrode layer 119 b functioningas a back gate electrode, the threshold voltage of the transistor 220can be shifted in the positive or negative direction. In the operationperiod of the semiconductor device, the transistor 220 can be changed toan enhancement-type or depression-type transistor, as appropriate byappropriate control of the threshold voltage of the transistor 220.

<Structure Example 4 of Transistor>

FIGS. 4A to 4B are a plan view and a cross-sectional view of atransistor 230 included in a semiconductor device of this embodiment.The transistor 230 is a modification example of the transistors 210 and220 in FIGS. 2A and 2B and FIGS. 3A and 3B. FIG. 4A is a plan view ofthe transistor 230, and FIG. 4B is a cross-sectional view taken alongdashed dotted lines A7-A8 and B7-B8 in FIG. 4A. Note that the substrate100 and some components (e.g., a gate insulating layer) of thetransistor 230 are not illustrated in FIG. 4A for simplicity.

The transistor 230 illustrated in FIGS. 4A and 4B includes the gateelectrode layer 102 formed over the substrate 100; the gate insulatinglayer 104 in contact with the gate electrode layer 102; the oxidesemiconductor layer 106 facing the gate electrode layer 102 with thegate insulating layer 104 positioned therebetween; the metal oxide layer108 functioning as a barrier layer and provided over the oxidesemiconductor layer 106; the pair of electrode layers 110 a and 110 b incontact with the metal oxide layer 108; the oxide insulating layer 112,the oxide insulating layer 114, and the nitride insulating layer 116formed over the pair of electrode layers 110 a and 110 b and the metaloxide layer 108; and the electrode layers 119 a and 119 c formed overthe nitride insulating layer 116.

The transistor 230 includes the electrode layers 119 a and 119 c, whichhave regions overlapping the oxide semiconductor layer 106 with themetal oxide layer 108 and the insulating layers (the oxide insulatinglayer 112, the oxide insulating layer 114, and the nitride insulatinglayer 116) positioned therebetween. The electrode layers 119 a and 119 care connected to the gate electrode layer 102 through the openingportions 117 a and 117 b, respectively, which are formed in the gateinsulating layer 104, the oxide insulating layer 112, the oxideinsulating layer 114, and the nitride insulating layer 116, and theelectrode layers 19 a and 119 c function as part of the gate electrodelayer 102. That is, the transistor 230 has the structure of thetransistor 220 in which the electrode layer 119 b functioning as a backgate electrode is omitted. Note that only one of the electrode layers119 a and 119 c may be provided in each of the transistor 220 and thetransistor 230.

The transistor 230 also includes the gate electrode layers (the gateelectrode layer 102 and the electrode layers 119 a and 119 c) that facethe top and bottom surfaces and two facing side surfaces of the oxidesemiconductor layer 206; therefore, like the transistors 210 and 220,the transistor 230 also has an s-channel structure in which the oxidesemiconductor layer 206 is electrically surrounded. Therefore, thecurrent drive capability of the transistor 230 is improved, so that thetransistor 230 can have high on-state current. The descriptions of thetransistors 210 and 220 can be referred to for details of everycomponents of the transistor 230.

Note that the structures of the transistors of this embodiment can befreely combined with each other.

<Method for Manufacturing Transistor>

A method for manufacturing the transistor of this embodiment isdescribed using FIGS. 5A to 5D and FIGS. 6A to 6C. Note that a methodfor manufacturing the transistor 210 is described below as a typicalexample.

First, a conductive film is formed over the substrate 100 and processedthrough a photolithography process to form the gate electrode layer 102.Next, the gate insulating layer 104 is formed over the gate electrodelayer 102 (see FIG. 5A).

The conductive film to be the gate electrode layer 102 can be formed bya sputtering method, a chemical vapor deposition (CVD) method, a vacuumevaporation method, or a pulsed laser deposition (PLD) method.Alternatively, a coating method or a printing method can be used.Although typical deposition methods are a sputtering method and a plasmachemical vapor deposition (PECVD) method, a thermal CVD method such as ametal organic chemical vapor deposition (MOCVD) method or an atomiclayer deposition (ALD) method may be used.

A thermal CVD method is a deposition method in which deposition may beperformed in such a manner that the pressure in a chamber is set to anatmospheric pressure or a reduced pressure, and a source gas and anoxidizer are supplied to the chamber at the same time and react witheach other in the vicinity of the substrate or over the substrate to bedeposited over the substrate. A thermal CVD method has an advantage thatno defect due to plasma damage is generated since it does not utilizeplasma for deposition.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a firstsingle-atomic layer; then the second source gas is introduced to reactwith the first single-atomic layer; as a result, a second single-atomiclayer is stacked over the first single-atomic layer, so that a thin filmis formed. The sequence of the gas introduction is repeated plural timesuntil a desired thickness is obtained, whereby a thin film withexcellent step coverage can be formed. The thickness of the thin filmcan be adjusted by the number of repetition times of the sequence of thegas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

In this embodiment, a glass substrate is used as the substrate 100, anda 100-nm-thick tungsten layer is formed as the gate electrode layer 102by a sputtering method.

Note that for example, in the case where a tungsten layer is formedusing a deposition apparatus utilizing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced a plurality of times to form an initial tungstenlayer, and then a WF₆ gas and an H₂ gas are introduced at a time, sothat a tungsten layer is formed. Note that an SiH₄ gas may be usedinstead of a B₂H₆ gas.

The gate insulating layer 104 can be formed by a sputtering method, aPECVD method, a thermal CVD method, a vacuum evaporation method, a PLDmethod, or the like. Here, a stack including a 400-nm-thick siliconnitride film and a 50-nm-thick silicon oxynitride film is formed as thegate insulating layer 104 by a PECVD method.

Alternatively, a film to be the gate insulating layer 104 may be formedby a thermal CVD method. For example, in the case where a hafnium oxidefilm is formed, two kinds of gases, i.e., ozone (O₃) as an oxidizer anda source gas which is obtained by vaporizing liquid containing a solventand a hafnium precursor compound (a hafnium alkoxide solution, typicallytetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemicalformula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples ofanother material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed, twokinds of gases, e.g., H₂O as an oxidizer and a source gas which isobtained by vaporizing liquid containing a solvent and an aluminumprecursor compound (e.g., trimethylaluminum (TMA)) are used. Note thatthe chemical formula of trimethylaluminum is Al(CH₃)₃. Examples ofanother material liquid include tris(dimethylamide)aluminum,triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed,hexachlorodisilane is adsorbed on a deposition surface, chlorinecontained in the adsorbate is removed, and radicals of an oxidizing gas(e.g., O₂ or dinitrogen monoxide) are supplied to react with theadsorbate.

Next, a stack including an oxide semiconductor film 106 a to be theoxide semiconductor layer 106 and a metal oxide film 108 a to be themetal oxide layer 108 is formed over the gate insulating layer 104 (seeFIG. 5B).

In this embodiment, an In—Ga—Zn oxide film is formed as the oxidesemiconductor film 106 a by a sputtering method using an In—Ga—Zn oxidetarget (In:Ga:Zn=1:1:1). Furthermore, an In—Ga oxide film is formed asthe metal oxide film 108 a by a sputtering method using an In—Ga oxidetarget (In:Ga=7:93). The metal oxide film 108 a is formed as an oxidesemiconductor film or an insulating film. Note that the constituentelements and compositions applicable to the oxide semiconductor film 106a and the metal oxide film 108 a are not limited thereto.

In the case where the oxide semiconductor film 106 a and the metal oxidefilm 108 a are formed by a sputtering method, a power supply device forgenerating plasma can be an RF power supply device, an AC power supplydevice, a DC power supply device, or the like as appropriate. Note thatit is preferable to use DC discharge applicable to a large-sizedsubstrate in deposition because the productivity of the semiconductordevice can be increased. To deposit the metal oxide film 108 a by asputtering method using DC discharge, it is preferable that y/(x+y) beless than or equal to 0.96, further preferably less than or equal to0.95, for example, 0.93 where an atomic ratio of In:M is x:y.

As a sputtering gas, a rare gas (typically argon), oxygen, or a mixedgas of a rare gas and oxygen is used as appropriate. In the case ofusing the mixed gas of a rare gas and oxygen, the proportion of oxygento a rare gas is preferably increased.

A chamber in a sputtering apparatus is preferably evacuated to be a highvacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with anadsorption vacuum evacuation pump such as a cryopump in order to removewater or the like, which serves as an impurity for the oxidesemiconductor film 106 a, as much as possible. Alternatively, a turbomolecular pump and a cold trap are preferably combined so as to preventa backflow of a gas, especially a gas containing carbon or hydrogen froman exhaust system to the inside of the chamber. It is preferable toremove impurities such as water contained in the metal oxide film 108 ain contact with the oxide semiconductor film 106 a, as much as possible;therefore, a chamber for depositing the metal oxide film 108 a ispreferably evacuated to be a high vacuum state.

In order to obtain a highly purified intrinsic or highly purifiedsubstantially intrinsic oxide semiconductor film, besides the highvacuum evacuation of the chamber, a highly purification of a sputteringgas is also needed. As an oxygen gas or an argon gas used for asputtering gas, a gas which is highly purified to have a dew point of−40° C. or lower, −80° C. or lower, −100° C. or lower, or −120° C. orlower is used, whereby entry of moisture or the like into the oxidesemiconductor film 106 a and the metal oxide film 108 a can beminimized.

Note that the oxide semiconductor film 106 a and/or the metal oxide film108 a can be formed with a deposition apparatus utilizing ALD instead ofsputtering. For example, in the case where an In—Ga—Zn oxide film isformed, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced pluraltimes to form an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas are introducedat a time to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anInGaO₂ layer, an InZnO₂ layer, a GaInO layer, a ZnInO layer, or a GaZnOlayer may be formed by mixing of these gases. Note that although an H₂Ogas which is obtained by bubbling with an inert gas such as Ar may beused instead of an O₃ gas, it is preferable to use an O₃ gas, which doesnot contain H. Further, instead of an In(GH₃)₃ gas, an In(C₂H₅)₃ gas maybe used. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ may be used. Insteadof a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂gas may be used.

Next, a resist mask is formed over the metal oxide film 108 a through aphotolithography process using a photoresist mask, and then the metaloxide film 108 a and the oxide semiconductor film 106 a are etched usingthe resist mask to be isolated for each element, so that the oxidesemiconductor layer 106 and the metal oxide layer 108 are formed (seeFIG. 5C). A wet etching method is preferably used for the etching. Notethat a dry etching method may be used, or a combination of both methodsmay be used.

After the oxide semiconductor layer 106 is formed, heat treatment may beperformed at a temperature higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 200° C. and lower than or equal to 450° C., further preferably higherthan or equal to 300° C. and lower than or equal to 450° C. The heattreatment performed here serves as one kind of treatment for increasingthe purity of the oxide semiconductor layer and can reduce hydrogen,water, and the like contained in the oxide semiconductor layer 106. Notethat the heat treatment for the purpose of reducing hydrogen, water, andthe like may be performed before the oxide semiconductor layer 106 isprocessed into an island shape. For example, the heat treatment may beperformed in a period from deposition of the oxide semiconductor film106 a to deposition of the metal oxide film 108 a. In this case, thedeposition temperature of the metal oxide film 108 a may be roomtemperature.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment performed on the oxide semiconductor layer 106. With theuse of an RTA apparatus, the heat treatment can be performed at atemperature higher than or equal to the strain point of the substrate ifthe heating time is short. Therefore, the heat treatment time can beshortened.

Note that the heat treatment performed on the oxide semiconductor layer106 may be performed under an atmosphere of nitrogen, oxygen, ultra-dryair (air with a water content of 20 ppm or less, preferably 1 ppm orless, more preferably 10 ppb or less), or a rare gas (argon, helium, orthe like). The atmosphere of nitrogen, oxygen, ultra-dry air, or a raregas preferably does not contain hydrogen, water, and the like. Further,after heat treatment performed in a nitrogen atmosphere or a rare gasatmosphere, heat treatment may be additionally performed in an oxygenatmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water,and the like can be released from the oxide semiconductor layer andoxygen can be supplied to the oxide semiconductor layer at the sametime. Consequently, the amount of oxygen vacancies in the oxidesemiconductor layer can be reduced.

Next, the pair of electrode layers 110 a and 110 b in contact with theside surfaces of the oxide semiconductor layer 106 and the side and topsurfaces of the metal oxide layer 108 is formed (see FIG. 5D).

Here, a 50-nm-thick tungsten film to be the first conductive layers 109a and 109 b and a 300-nm-thick copper film to be the second conductivelayers 111 a and 111 b are formed by a sputtering method. Then, a resistmask is formed over the copper film through a photolithography processusing a photoresist mask, and the tungsten film and the copper film areprocessed using the resist mask to be the pair of electrode layers 110 aand 110 b. Note that as described above, the conductive films such asthe tungsten film and the copper film may be formed by an ALD method ora thermal CVD method. Any of these methods makes it possible to form theconductive films without plasma damage to the oxide semiconductor layer106 and the metal oxide layer 108.

For example, when a wet etching method is used for etching the copperfilm and a dry etching method using SF₆ is used for etching the tungstenfilm, a fluoride is formed on the surface of the copper film, and copperof the copper film can be prevented from diffusing to the oxidesemiconductor layer 106 owing to the fluoride. In addition, the metaloxide layer 108 can function as an etching protective film for the oxidesemiconductor layer 106.

Then, the oxide insulating layer 112 is formed over the pair ofelectrode layers 110 a and 110 b. After that, the oxide insulating layer114 is formed over the oxide insulating layer 112 (see FIG. 6A).

It is preferable to form the oxide insulating layer 114 without exposureto the atmosphere, directly after the oxide insulating layer 112 isformed. After the oxide insulating layer 112 is formed, the oxideinsulating layer 114 is formed by adjusting at least one of the flowrate of a source gas, pressure, a high-frequency power, and a substratetemperature without exposure to the air, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the oxide insulating layer 112 and the oxide insulating layer114 can be reduced and oxygen in the oxide insulating layer 114 can bemoved to the oxide semiconductor layer 106; accordingly, the amount ofoxygen vacancy in the oxide semiconductor layer 106 can be reduced.

For example, a silicon oxide film or a silicon oxynitride film can beformed as the oxide insulating layer 112 under the following conditions;the substrate placed in an evacuated treatment chamber of the plasma CVDapparatus is held at a temperature ranging from 180° C. to 400° C.,preferably from 200° C. to 370° C.; the pressure of the chamber intowhich the source gas is introduced is set in the range from 20 Pa to 250Pa, preferably from 100 Pa to 250 Pa; and high-frequency power issupplied to the electrode provided in the treatment chamber.

With the use of the above deposition conditions, an oxide insulatinglayer which is permeable to oxygen can be formed as the oxide insulatinglayer 112. Further, by providing the metal oxide layer 108 and the oxideinsulating layer 112, damage to the oxide semiconductor layer 106 can bereduced in a step of forming the oxide insulating layer 114 which isformed later.

Under these deposition conditions, the bonding strength of silicon andoxygen becomes strong when the substrate temperature is higher than orequal to 300° C. and lower than or equal to 400° C., preferably higherthan or equal to 320° C. and lower than or equal to 370° C. Thus, as theoxide insulating layer 112, a dense and hard oxide insulating layer thatis permeable to oxygen, typically, a silicon oxide film or a siliconoxynitride film of which etching using hydrofluoric acid of 0.5 wt % at25° C. is performed at a rate of lower than or equal to 10 nm/min,preferably lower than or equal to 8 nm/min can be formed.

It is effective for release of hydrogen, water, and the like containedin the oxide semiconductor layer 106 and the metal oxide layer 108 incontact therewith to form the oxide insulating layer 112 while heatingis performed in the step of depositing the oxide insulating layer 112.Hydrogen contained in the oxide semiconductor layer 106 is bonded to anoxygen radical formed in plasma to form water. Since the substrate isheated in the step for depositing the oxide insulating layer 112, waterformed by bonding of oxygen and hydrogen is released from the oxidesemiconductor layer 106. That is, formation of the oxide insulatinglayer 112 by a plasma CVD method can reduce the amount of water,hydrogen, and the like contained in the oxide semiconductor layer 106.

Furthermore, by setting the pressure in the treatment chamber to behigher than or equal to 100 Pa and lower than or equal to 250 Pa, theamount of water contained in the oxide insulating layer 112 is reduced;thus, variation in electrical characteristics of the transistor 210 canbe reduced and change in threshold voltage can be inhibited.

Note that it is preferable to reduce damage to the oxide semiconductorlayer 106 as much as possible at the time of depositing the oxideinsulating layer 112. This is because in the case where the oxideinsulating layer 114 that is formed later for the purpose of improvingthe reliability of the transistor is formed under the conditions thatcan reduce the defects in the film, the amount of oxygen released fromthe oxide insulating layer 114 tends to be reduced, and thus it isdifficult to adequately reduce defects of the oxide semiconductor layer106. Thus, it is preferable that the pressure in a treatment chamber behigher than or equal to 100 Pa and lower than or equal to 250 Pa at thetime of depositing the oxide insulating layer 112. Deposition under suchconditions can reduce damage to the oxide semiconductor layer 106.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the oxide insulating layer 112 can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorlayer 106 can be reduced, thereby inhibiting the negative shift in thethreshold voltage of the transistor.

As the oxide insulating layer 114, a silicon oxide film or a siliconoxynitride film is formed under the following conditions; the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to the electrodeprovided in the treatment chamber.

As the deposition conditions of the oxide insulating layer 114, thehigh-frequency power having the above power density is supplied to thereaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content of the oxide insulating layer 114 becomes higher thanthat in the stoichiometric composition. On the other hand, in the filmformed at a substrate temperature within the above temperature range, abond between silicon and oxygen is weak, and accordingly, part of oxygenin the film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating layer which contains oxygen in aproportion higher than that of oxygen in the stoichiometric compositionand from which part of oxygen is released by heating.

Note that the oxide insulating layer 112 serves as a protective film ofthe metal oxide layer 108 in the step of forming the oxide insulatinglayer 114. Furthermore, the metal oxide layer 108 serves as a protectivefilm of the oxide semiconductor layer 106. Consequently, the oxideinsulating layer 114 can be formed using the high-frequency power havinga high power density while damage to the oxide semiconductor layer 106is reduced.

Note that in the deposition conditions of the oxide insulating layer114, when the flow rate of the deposition gas containing silicon withrespect to the oxidizing gas is increased, the amount of defects in theoxide insulating layer 114 can be reduced. Typically, it is possible toform an oxide insulating layer in which the amount of defects is small,i.e., the spin density of a signal which appears at g=2.001 originatingfrom a dangling bond of silicon is lower than 6×10¹⁷ spins/cm³,preferably lower than or equal to 3×10¹⁷ spins/cm³, further preferablylower than or equal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As aresult, the reliability of the transistor can be improved.

After the oxide insulating layers 112 and 114 are formed, heat treatmentis performed. By the heat treatment, part of oxygen contained in theoxide insulating layer 114 can be moved to the oxide semiconductor layer106, so that the amount of oxygen vacancy contained in the oxidesemiconductor layer 106 can be further reduced. After the heattreatment, the nitride insulating layer 116 is formed.

In the case where water, hydrogen, or the like is contained in the oxideinsulating layers 112 and 114, when the nitride insulating layer 116having a function of blocking water, hydrogen, and the like is formedand then heat treatment is performed, water, hydrogen, or the likecontained in the oxide insulating layers 112 and 114 are moved to theoxide semiconductor layer 106, so that defects are generated in theoxide semiconductor layer 106. Thus, when heat treatment is performedbefore formation of the nitride insulating layer 116, water or hydrogencontained in the oxide insulating layers 112 and 114 can be effectivelyreduced.

Note that when the oxide insulating layer 114 is formed over the oxideinsulating layer 112 while being heated, oxygen can be moved to theoxide semiconductor layer 106 to reduce oxygen vacancy included in theoxide semiconductor layer 106; therefore, the heat treatment is notnecessarily performed in some cases.

The temperature of the heat treatment performed on the oxide insulatinglayers 112 and 114 is typically higher than or equal to 150° C. andlower than or equal to 400° C. preferably higher than or equal to 300°C. and lower than or equal to 400° C. further preferably higher than orequal to 320° C. and lower than or equal to 370° C. The heat treatmentmay be performed in an atmosphere of nitrogen, oxygen, ultra-dry air(air in which a water content is 20 ppm or less, preferably 1 ppm orless, further preferably 10 ppb or less), or a rare gas (argon, helium,or the like). Note that an electric furnace, an RTA apparatus, or thelike can be used for the heat treatment, in which it is preferable thathydrogen, water, and the like not be contained in the nitrogen, oxygen,ultra-dry air, or rare gas.

Here, the heat treatment is performed at 350° C. in a mixed atmosphereof nitrogen and oxygen for one hour. After that, the nitride insulatinglayer 116 is formed (see FIG. 6A).

In the case where the nitride insulating layer 116 is formed by a plasmaCVD method, the substrate temperature is preferably higher than or equalto 300° C. and lower than or equal to 400° C., further preferably higherthan or equal to 320° C. and lower than or equal to 370° C. because adense film can be formed.

For example, in the case where a silicon nitride film is formed as thenitride insulating layer 116 by the plasma CVD method, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used as asource gas. A small amount of ammonia compared to the amount of nitrogenis used, whereby ammonia is dissociated in plasma and activated speciesare generated. The activated species cleave a bond between silicon andhydrogen which are contained in a deposition gas containing silicon anda triple bond between nitrogen molecules. As a result, a dense siliconnitride film having few defects, in which bonds between silicon andnitrogen are promoted and bonds between silicon and hydrogen is few, canbe formed. On the other hand, when the amount of ammonia with respect tonitrogen is large, decomposition of a deposition gas containing siliconand decomposition of nitrogen are not promoted, so that a sparse siliconnitride film in which bonds between silicon and hydrogen remain anddefects are increased is formed. Therefore, in the source gas, a flowrate ratio of the nitrogen to the ammonia is set to be greater than orequal to 5 and less than or equal to 50, preferably greater than orequal to 10 and less than or equal to 50.

Here, a 50-nm-thick silicon nitride film is formed as the nitrideinsulating layer 116 using source gases of silane, nitrogen, and ammoniawith a plasma CVD apparatus. The flow rates of silane, nitrogen, andammonia are 50 sccm, 5000 sccm, and 100 sccm, respectively. The pressurein a treatment chamber is set to 100 Pa, the substrate temperature isset to 350° C., and a high frequency power of 1000 W is supplied toparallel plate electrodes using a high frequency power source of 27.12MHz. Note that a PECVD apparatus is a parallel-plate plasma CVDapparatus in which the electrode area is 6000 cm², and the power perunit area (power density) into which the supplied power is converted is1.7×10⁻¹ W/cm².

After formation of the nitride insulating layer 116, heat treatment maybe performed. The heat treatment is performed typically at a temperaturehigher than or equal to 150° C. and lower than or equal to 400° C.,preferably higher than or equal to 300° C. and lower than or equal to400° C., further preferably higher than or equal to 320° C. and lowerthan or equal to 370° C. When the heat treatment is performed, theamount of hydrogen and water of the oxide insulating layers 112 and 114is reduced; therefore, generation of defects in the oxide semiconductorlayer 106 described above is inhibited.

Next, a resist mask is formed over the nitride insulating layer 116through a photolithography process using a photoresist mask. The nitrideinsulating layer 116, the oxide insulating layers 112 and 114, and thegate insulating layer 104 are etched using the resist mask to form theopening portions 17 a and 117 b (see FIG. 6B).

After the resist mask is removed, a conductive film is formed over thenitride insulating layer 116 and processed to form the electrode layer118 functioning as a back gate electrode (see FIG. 6C).

Through the above-described process, the transistor 210 of thisembodiment can be formed. Note that the other transistors of thisembodiment can be formed in a manner similar to that of the transistor210.

As described above, since electrode layers containing a low-resistancematerial such as copper, aluminum, gold, or silver are used as the pairof electrode layers functioning as the source and drain electrode layersin the transistor described in this embodiment, a semiconductor devicein which wiring delay is suppressed can be manufactured. Furthermore,when a metal oxide layer functioning as a barrier layer is provided incontact with the pair of electrode layers, a reduction in the electricalcharacteristics can be inhibited, so that the semiconductor device canhave favorable electrical characteristics.

According to the manufacturing process of this embodiment, it ispossible to manufacture a highly reliable transistor in which the oxygenvacancy in the oxide semiconductor layer including the channel formationregion is reduced and the impurity concentration is reduced.

Furthermore, since the transistor of this embodiment is a channel-etchedtransistor that is formed in such a manner that the metal oxide layer108 functioning as a barrier layer for preventing entry of impurities isformed using the same mask as the oxide semiconductor layer 106, thenumber of masks can be reduced as compared to the case of a channelprotective transistor. Therefore, the manufacturing cost of thesemiconductor device can be reduced.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 2

The structure of an oxide semiconductor layer included in a transistorof one embodiment of the present invention is described in thisembodiment.

A structure which can be included in an oxide semiconductor layer isdescribed below.

An oxide semiconductor layer is classified into, for example, anon-single-crystal oxide semiconductor layer and a single crystal oxidesemiconductor layer. Alternatively, an oxide semiconductor layer isclassified into, for example, a crystalline oxide semiconductor layerand an amorphous oxide semiconductor layer.

Examples of a non-single-crystal oxide semiconductor include a c-axisaligned crystalline oxide semiconductor (CAAC-OS), a polycrystallineoxide semiconductor, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. In addition, examples of a crystallineoxide semiconductor include a single crystal oxide semiconductor, aCAAC-OS, a polycrystalline oxide semiconductor, and a microcrystallineoxide semiconductor.

First, a CAAC-OS layer is described.

A CAAC-OS layer is one of oxide semiconductor layers having a pluralityof c-axis aligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

FIG. 8A shows an example of a high-resolution TEM image of a crosssection of the CAAC-OS which is obtained from a direction substantiallyparallel to the sample surface. Here, the TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image in the followingdescription. Note that the Cs-corrected high-resolution TEM image can beobtained with, for example, an atomic resolution analytical electronmicroscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 8B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 8A. FIG. 8B shows that metal atoms are arranged in alayered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 8B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 8C. FIGS. 8B and 8C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 8D). The part in which the pellets are tilted as observed inFIG. 8C corresponds to a region 5161 shown in FIG. 8D.

For example, as shown in FIG. 23A, a Cs-corrected high-resolution TEMimage of a plane of the CAAC-OS obtained from a direction substantiallyperpendicular to the sample surface is observed. FIGS. 23B, 23C, and 23Dare enlarged Cs-corrected high-resolution TEM images of regions (1),(2), and (3) in FIG. 23A, respectively. FIGS. 23B, 23C, and 23D indicatethat metal atoms are arranged in a triangular, quadrangular, orhexagonal configuration in a pellet. However, there is no regularity ofarrangement of metal atoms between different pellets.

For example, when the structure of a CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method using an X-ray diffraction(XRD) apparatus, a peak appears at a diffraction angle (2θ) of around31° as shown in FIG. 24A. This peak is derived from the (009) plane ofthe InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS havec-axis alignment, and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS.

Note that in structural analysis of the CAAC-OS including an InGaZnO₄crystal by an out-of-plane method, another peak may appear when 2θaround 36°, in addition to the peak at 2θ of around 31°. The peak at 2θof around 36° indicates that a crystal having no c-axis alignment isincluded in part of the CAAC-OS. It is preferable that in the CAAC-OS, apeak appear when 2θ is around 31° and that a peak not appear when 2θ isaround 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (φ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (φ axis), as shown in FIG. 24B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when φ scan is performed with2θ fixed at around 56°, as shown in FIG. 24C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are different in the CAAC-OS.

Next, FIG. 25A shows a diffraction pattern (also referred to as aselected-area transmission electron diffraction pattern) obtained insuch a manner that an electron beam with a probe diameter of 300 nm isincident on an In—Ga—Zn oxide that is a CAAC-OS in a direction parallelto the sample surface. As shown in FIG. 25A, for example, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are observed. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 25B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 25B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 25B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 25B is considered to be derived from the (110)plane and the like.

Since the c-axes of the pellets (nanocrystals) are aligned in adirection substantially perpendicular to the formation surface or thetop surface in the above manner, the CAAC-OS can also be referred to asan oxide semiconductor including c-axis aligned nanocrystals (CANC).

The CAAC-OS is an oxide semiconductor with a low impurity concentration.The impurity means an element other than the main components of theoxide semiconductor, such as hydrogen, carbon, silicon, or a transitionmetal element. An element (specifically, silicon or the like) havinghigher strength of bonding to oxygen than a metal element included in anoxide semiconductor extracts oxygen from the oxide semiconductor, whichresults in disorder of the atomic arrangement and reduced crystallinityof the oxide semiconductor. A heavy metal such as iron or nickel, argon,carbon dioxide, or the like has a large atomic radius (or molecularradius), and thus disturbs the atomic arrangement of the oxidesemiconductor and decreases crystallinity. Additionally, the impuritycontained in the oxide semiconductor might serve as a carrier trap or acarrier generation source.

Moreover, the CAAC-OS is an oxide semiconductor having a low density ofdefect states. For example, oxygen vacancies in the oxide semiconductorserve as carrier traps or serve as carrier generation sources whenhydrogen is captured therein.

In a transistor using the CAAC-OS, change in electrical characteristicsdue to irradiation with visible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor layer is described.

A microcrystalline oxide semiconductor has a region in which a crystalpart is observed and a region in which a crystal part is not clearlyobserved in a high-resolution TEM image. In most cases, the size of acrystal part included in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. An oxidesemiconductor including a nanocrystal that is a microcrystal with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, or a sizegreater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as a nanocrystalline oxide semiconductor(nc-OS). In a high-resolution TEM image of the nc-OS, for example, agrain boundary is not clearly observed in some cases. Note that there isa possibility that the origin of the nanocrystal is the same as that ofa pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may bereferred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an amorphous oxidesemiconductor, depending on an analysis method. For example, when thenc-OS is subjected to structural analysis by an out-of-plane method withan XRD apparatus using an X-ray having a diameter larger than the sizeof a pellet, a peak which shows a crystal plane does not appear.Furthermore, a diffraction pattern like a halo pattern is observed whenthe nc-OS is subjected to electron diffraction using an electron beamwith a probe diameter (e.g., 50 nm or larger) that is larger than thesize of a pellet (the electron diffraction is also referred to asselected-area electron diffraction). Meanwhile, spots appear in ananobeam electron diffraction pattern of the nc-OS when an electron beamhaving a probe diameter close to or smaller than the size of a pellet isapplied. Moreover, in a nanobeam electron diffraction pattern of thenc-OS, regions with high luminance in a circular (ring) pattern areshown in some cases. Also in a nanobeam electron diffraction pattern ofthe nc-OS, a plurality of spots is shown in a ring-like region in somecases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an amorphous oxidesemiconductor. Note that there is no regularity of crystal orientationbetween different pellets in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the CAAC-OS.

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is an oxide semiconductor havingdisordered atomic arrangement and no crystal part and exemplified by anoxide semiconductor which exists in an amorphous state as quartz.

In a high-resolution TEM image of the amorphous oxide semiconductor,crystal parts cannot be found.

When the amorphous oxide semiconductor is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor is subjected to electron diffraction.Furthermore, a spot is not observed and a halo pattern appears when theamorphous oxide semiconductor is subjected to nanobeam electrondiffraction.

There are various understandings of an amorphous structure. For example,a structure whose atomic arrangement does not have ordering at all iscalled a completely amorphous structure. Meanwhile, a structure whichhas ordering until the nearest neighbor atomic distance or thesecond-nearest neighbor atomic distance but does not have long-rangeordering is also called an amorphous structure. Therefore, the strictestdefinition does not permit an oxide semiconductor to be called anamorphous oxide semiconductor as long as even a negligible degree ofordering is present in an atomic arrangement. At least an oxidesemiconductor having long-term ordering cannot be called an amorphousoxide semiconductor. Accordingly, because of the presence of crystalpart, for example, a CAAC-OS and an nc-OS cannot be called an amorphousoxide semiconductor or a completely amorphous oxide semiconductor.

Note that an oxide semiconductor may have a structure having physicalproperties intermediate between the nc-OS and the amorphous oxidesemiconductor. The oxide semiconductor having such a structure isspecifically referred to as an amorphous-like oxide semiconductor(a-like OS).

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

A difference in effect of electron irradiation between structures of anoxide semiconductor is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared. Each of the samplesis an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Then, the size of the crystal part of each sample is measured. FIG. 26shows the change in the average size of crystal parts (at 22 points to45 points) in each sample. FIG. 26 indicates that the crystal part sizein the a-like OS increases with an increase in the cumulative electrondose. Specifically, as shown by (1) in FIG. 26, a crystal part ofapproximately 1.2 nm at the start of TEM observation (the crystal partis also referred to as an initial nucleus) grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm² regardless of the cumulative electrondose. Specifically, as shown by (2) in FIG. 26, the average crystal sizeis approximately 1.4 nm regardless of the observation time by TEM.Furthermore, as shown by (3) in FIG. 26, the average crystal size isapproximately 2.1 nm regardless of the observation time by TEM.

In this manner, growth of the crystal part occurs due to thecrystallization of the a-like OS, which is induced by a slight amount ofelectron beam employed in the TEM observation. In contrast, in the nc-OSand the CAAC-OS that have good quality, crystallization hardly occurs bya slight amount of electron beam used for TEM observation.

Note that the crystal part size in the a-like OS and the nc-OS can bemeasured using high-resolution TEM images. For example, an InGaZnO₄crystal has a layered structure in which two Ga—Zn—O layers are includedbetween In—O layers. A unit cell of the InGaZnO₄ crystal has a structurein which nine layers including three In—O layers and six Ga—Zn—O layersare stacked in the c-axis direction. Accordingly, the distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Thus, focusing on lattice fringesin the high-resolution TEM image, each of lattice fringes in which thelattice spacing therebetween is greater than or equal to 0.28 nm andless than or equal to 0.30 nm corresponds to the a-b plane of theInGaZnO₄ crystal.

Furthermore, the density of an oxide semiconductor varies depending onthe structure in some cases. For example, when the composition of anoxide semiconductor is determined, the structure of the oxidesemiconductor can be expected by comparing the density of the oxidesemiconductor with the density of a single crystal oxide semiconductorhaving the same composition as the oxide semiconductor. For example, thedensity of the a-like OS is higher than or equal to 78.6% and lower than92.3% of the density of the single crystal oxide semiconductor havingthe same composition. For example, the density of each of the nc-OS andthe CAAC-OS is higher than or equal to 92.3% and lower than 100% of thedensity of the single crystal oxide semiconductor having the samecomposition. Note that it is difficult to deposit an oxide semiconductorhaving a density of lower than 78% of the density of the single crystaloxide semiconductor.

Specific examples of the above description are given. For example, inthe case of an oxide semiconductor having an atomic ratio ofIn:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

Note that an oxide semiconductor may be a stacked film including two ormore films of an amorphous oxide semiconductor, an a-like OS, amicrocrystalline oxide semiconductor, and a CAAC-OS, for example.

An oxide semiconductor having a low impurity concentration and a lowdensity of defect states (a small number of oxygen vacancies) can havelow carrier density. Therefore, such an oxide semiconductor is referredto as a highly purified intrinsic or highly purified substantiallyintrinsic oxide semiconductor. A CAAC-OS and an nc-OS have a lowimpurity concentration and a low density of defect states as compared toan a-like OS and an amorphous oxide semiconductor. That is, a CAAC-OSand an nc-OS are likely to be highly purified intrinsic or highlypurified substantially intrinsic oxide semiconductors. Thus, atransistor including a CAAC-OS or an nc-OS rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic or highlypurified substantially intrinsic oxide semiconductor has few carriertraps. Therefore, a transistor including a CAAC-OS or an nc-OS has smallvariation in electrical characteristics and high reliability. Anelectric charge trapped by the carrier traps in the oxide semiconductortakes a long time to be released. The trapped electric charge may behavelike a fixed electric charge. Thus, the transistor which includes theoxide semiconductor having a high impurity concentration and a highdensity of defect states might have unstable electrical characteristics.

<Deposition Model>

Examples of deposition models of a CAAC-OS and an nc-OS are describedbelow.

FIG. 27A is a schematic view of the inside of a deposition chamber wherea CAAC-OS is deposited by a sputtering method.

A target 5130 is attached to a backing plate. A plurality of magnets isprovided to face the target 5130 with the backing plate positionedtherebetween. The plurality of magnets generates a magnetic field. Asputtering method in which the disposition rate is increased byutilizing a magnetic field of magnets is referred to as a magnetronsputtering method.

The target 5130 has a polycrystalline structure in which a cleavageplane exists in at least one crystal grain.

A cleavage plane of the target 5130 including an In—Ga—Zn oxide isdescribed as an example. FIG. 28A shows a structure of an InGaZnO₄crystal included in the target 5130. Note that FIG. 28A shows astructure of the case where the InGaZnO₄ crystal is observed from adirection parallel to the b-axis when the c-axis is in an upwarddirection.

FIG. 28A indicates that oxygen atoms in a Ga—Zn—O layer are positionedclose to those in an adjacent Ga—Zn—O layer. The oxygen atoms havenegative charge, whereby the two Ga—Zn—O layers repel each other. As aresult, the InGaZnO₄ crystal has a cleavage plane between the twoadjacent Ga—Zn—O layers.

The substrate 5120 is placed to face the target 5130, and the distance d(also referred to as a target-substrate distance (T-S distance)) isgreater than or equal to 0.01 m and less than or equal to 1 m,preferably greater than or equal to 0.02 m and less than or equal to 0.5m. The deposition chamber is mostly filled with a deposition gas (e.g.,an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol %or higher) and the pressure in the deposition chamber is controlled tobe higher than or equal to 0.01 Pa and lower than or equal to 100 Pa,preferably higher than or equal to 0.1 Pa and lower than or equal to 10Pa. Here, discharge starts by application of a voltage at a certainvalue or higher to the target 5130, and plasma is observed. The magneticfield forms a high-density plasma region in the vicinity of the target5130. In the high-density plasma region, the deposition gas is ionized,so that an ion 5101 is generated. Examples of the ion 5101 include anoxygen cation (O⁺) and an argon cation (Ar⁺).

The ion 5101 is accelerated toward the target 5130 side by an electricfield, and then collides with the target 5130. At this time, a pellet5100 a and a pellet 5100 b which are flat-plate-like (pellet-like)sputtered particles are separated and sputtered from the cleavage plane.Note that structures of the pellet 5100 a and the pellet 5100 b may bedistorted by an impact of collision of the ion 5101.

The pellet 5100 a is a flat-plate-like (pellet-like) sputtered particlehaving a triangle plane, e.g., regular triangle plane. The pellet 5100 bis a flat-plate-like (pellet-like) sputtered particle having a hexagonplane, e.g., regular hexagon plane. Note that flat-plate-like(pellet-like) sputtered particles such as the pellet 5100 a and thepellet 5100 b are collectively called pellets 5100. The shape of a flatplane of the pellet 5100 is not limited to a triangle or a hexagon. Forexample, the flat plane may have a shape formed by combining two or moretriangles. For example, a quadrangle (e.g., rhombus) may be formed bycombining two triangles (e.g., regular triangles).

The thickness of the pellet 5100 is determined depending on the kind ofdeposition gas and the like. The thicknesses of the pellets 5100 arepreferably uniform; the reason for this is described later. In addition,the sputtered particle preferably has a pellet shape with a smallthickness as compared to a dice shape with a large thickness. Forexample, the thickness of the pellet 5100 is greater than or equal to0.4 nm and less than or equal to 1 nm, preferably greater than or equalto 0.6 nm and less than or equal to 0.8 nm. In addition, for example,the width of the pellet 5100 is greater than or equal to 1 nm and lessthan or equal to 3 nm, preferably greater than or equal to 1.2 nm andless than or equal to 2.5 nm. The pellet 5100 corresponds to the initialnucleus in the description of (1) in FIG. 26. For example, in the casewhere the ion 5101 collides with the target 5130 including an In—Ga—Znoxide, the pellet 5100 that includes three layers of a Ga—Zn—O layer, anIn—O layer, and a Ga—Zn—O layer as shown in FIG. 28B is ejected. Notethat FIG. 28C shows the structure of the pellet 5100 observed from adirection parallel to the c-axis. Therefore, the pellet 5100 has ananometer-sized sandwich structure including two Ga—Zn—O layers (piecesof bread) and an In—O layer (filling).

The pellet 5100 may receive a charge when passing through the plasma, sothat side surfaces thereof are negatively or positively charged. Thepellet 5100 includes an oxygen atom on its side surface, and the oxygenatom may be negatively charged. In this manner, when the side surfacesare charged with the same polarity, charges repel each other, andaccordingly, the pellet 5100 can maintain a flat-plate shape. In thecase where a CAAC-OS is an In—Ga—Zn oxide, there is a possibility thatan oxygen atom bonded to an indium atom is negatively charged. There isanother possibility that an oxygen atom bonded to an indium atom, agallium atom, or a zinc atom is negatively charged. In addition, thepellet 5100 may grow by being bonded with an indium atom, a galliumatom, a zinc atom, an oxygen atom, or the like when passing throughplasma. A difference in size between (2) and (1) in FIG. 26 correspondsto the amount of growth in plasma. Here, in the case where thetemperature of the substrate 5120 is at around room temperature, thepellet 5100 does not grow anymore; thus, an nc-OS is formed (see FIG.27B). An nc-OS can be deposited when the substrate 5120 has a large sizebecause a temperature at which the deposition of an nc-OS is carried outis approximately room temperature. Note that in order that the pellet5100 grows in plasma, it is effective to increase deposition power insputtering. High deposition power can stabilize the structure of thepellet 5100.

As shown in FIGS. 27A and 27B, the pellet 5100 flies like a kite inplasma and flutters up to the substrate 5120. Since the pellets 5100 arecharged, when the pellet 5100 gets close to a region where anotherpellet 5100 has already been deposited, repulsion is generated. Here,above the substrate 5120, a magnetic field in a direction parallel tothe top surface of the substrate 5120 (also referred to as a horizontalmagnetic field) is generated. A potential difference is given betweenthe substrate 5120 and the target 5130, and accordingly, current flowsfrom the substrate 5120 toward the target 5130. Thus, the pellet 5100 isgiven a force (Lorentz force) on the top surface of the substrate 5120by an effect of the magnetic field and the current. This is explainablewith Fleming's left-hand rule.

The mass of the pellet 5100 is larger than that of an atom. Therefore,to move the pellet 5100 over the top surface of the substrate 5120, itis important to apply some force to the pellet 5100 from the outside.One kind of the force may be force which is generated by the action of amagnetic field and current. In order to increase a force applied to thepellet 5100, it is preferable to provide, on the top surface, a regionwhere the magnetic field in a direction parallel to the top surface ofthe substrate 5120 is 10 G or higher, preferably 20 G or higher, furtherpreferably 30 G or higher, still further preferably 50 G or higher.Alternatively, it is preferable to provide, on the top surface, a regionwhere the magnetic field in a direction parallel to the top surface ofthe substrate 5120 is 1.5 times or higher, preferably twice or higher,further preferably 3 times or higher, still further preferably 5 timesor higher as high as the magnetic field in a direction perpendicular tothe top surface of the substrate 5120.

At this time, the magnets and the substrate 5120 are moved or rotatedrelatively, whereby the direction of the horizontal magnetic field onthe top surface of the substrate 5120 continues to change. Therefore,the pellet 5100 can be moved in various directions on the top surface ofthe substrate 5120 by receiving forces in various directions.

Furthermore, as shown in FIG. 27A, when the substrate 5120 is heated,resistance between the pellet 5100 and the substrate 5120 due tofriction or the like is low. As a result, the pellet 5100 glides abovethe top surface of the substrate 5120. The glide of the pellet 5100 iscaused in a state where its flat plane faces the substrate 5120. Then,when the pellet 5100 reaches the side surface of another pellet 5100that has been already deposited, the side surfaces of the pellets 5100are bonded. At this time, the oxygen atom on the side surface of thepellet 5100 is released. With the released oxygen atom, oxygen vacanciesin a CAAC-OS might be filled; thus, the CAAC-OS has a low density ofdefect states. Note that the temperature of the top surface of thesubstrate 5120 is, for example, higher than or equal to 100° C. andlower than 500° C., higher than or equal to 150° C. and lower than 450°C., or higher than or equal to 170° C. and lower than 400° C. Hence,even when the substrate 5120 has a large size, it is possible to deposita CAAC-OS.

Furthermore, the pellet 5100 is heated on the substrate 5120, wherebyatoms are rearranged, and the structure distortion caused by thecollision of the ion 5101 can be reduced. The pellet 5100 whosestructure distortion is reduced is substantially single crystal. Evenwhen the pellets 5100 are heated after being bonded, expansion andcontraction of the pellet 5100 itself hardly occur, which is caused byturning the pellet 5100 into substantially single crystal. Thus,formation of defects such as a grain boundary due to expansion of aspace between the pellets 5100 can be prevented, and accordingly,generation of crevasses can be prevented.

The CAAC-OS does not have a structure like a board of a single crystaloxide semiconductor but has arrangement with a group of pellets 5100(nanocrystals) like stacked bricks or blocks. Furthermore, a grainboundary does not exist therebetween. Therefore, even when deformationsuch as shrink occurs in the CAAC-OS owing to heating during deposition,heating or bending after deposition, it is possible to relieve localstress or release distortion. Therefore, this structure is suitable fora flexible semiconductor device. Note that the nc-OS has arrangement inwhich pellets 5100 (nanocrystals) are randomly stacked.

When the target is sputtered with an ion, in addition to the pellets,zinc oxide or the like may be ejected. The zinc oxide is lighter thanthe pellet and thus reaches the top surface of the substrate 5120 beforethe pellet. As a result, the zinc oxide forms a zinc oxide layer 5102with a thickness greater than or equal to 0.1 nm and less than or equalto 10 nm, greater than or equal to 0.2 nm and less than or equal to 5nm, or greater than or equal to 0.5 nm and less than or equal to 2 nm.FIGS. 29A to 29D are cross-sectional schematic views.

As illustrated in FIG. 29A, a pellet 5105 a and a pellet 5105 b aredeposited over the zinc oxide layer 5102. Here, side surfaces of thepellet 5105 a and the pellet 5105 b are in contact with each other. Inaddition, a pellet 5105 c is deposited over the pellet 5105 b, and thenglides over the pellet 5105 b. Furthermore, a plurality of particles5103 ejected from the target together with the zinc oxide iscrystallized by heating of the substrate 5120 to form a region 5105 a 1on another side surface of the pellet 5105 a. Note that the plurality ofparticles 5103 may contain oxygen, zinc, indium, gallium, or the like.

Then, as illustrated in FIG. 29B, the region 5105 a 1 grows to part ofthe pellet 5105 a to form a pellet 5105 a 2. In addition, a side surfaceof the pellet 5105 c is in contact with another side surface of thepellet 5105 b.

Next, as illustrated in FIG. 29C, a pellet 5105 d is deposited over thepellet 5105 a 2 and the pellet 5105 b, and then glides over the pellet5105 a 2 and the pellet 5105 b. Furthermore, a pellet 5105 e glidestoward another side surface of the pellet 5105 c over the zinc oxidelayer 5102.

Then, as illustrated in FIG. 29D, the pellet 5105 d is placed so that aside surface of the pellet 5105 d is in contact with a side surface ofthe pellet 5105 a 2. Furthermore, a side surface of the pellet 5105 e isin contact with another side surface of the pellet 5105 c. A pluralityof particles 5103 ejected from the target together with the zinc oxideis crystallized by heating of the substrate 5120 to form a region 5105 d1 on another side surface of the pellet 5105 d.

As described above, deposited pellets are placed to be in contact witheach other and then growth is caused at side surfaces of the pellets,whereby a CAAC-OS is formed over the substrate 5120. Therefore, eachpellet of the CAAC-OS is larger than that of the nc-OS. A difference insize between (3) and (2) in FIG. 26 corresponds to the amount of growthafter deposition.

When spaces between pellets 5100 are extremely small, the pellets mayform a large pellet. The large pellet has a single crystal structure.For example, the size of the large pellet may be greater than or equalto 10 nm and less than or equal to 200 nm, greater than or equal to 15nm and less than or equal to 100 nm, or greater than or equal to 20 nmand less than or equal to 50 nm, when seen from the above. Therefore,when a channel formation region of a transistor is smaller than thelarge pellet, the region having a single crystal structure can be usedas the channel formation region. Furthermore, when the size of thepellet is increased, the region having a single crystal structure can beused as the channel formation region, the source region, and the drainregion of the transistor.

In this manner, when the channel formation region or the like of thetransistor is formed in a region having a single crystal structure, thefrequency characteristics of the transistor can be increased in somecases.

As shown in such a model, the pellets 5100 are considered to bedeposited on the substrate 5120. Thus, a CAAC-OS can be deposited evenwhen a formation surface does not have a crystal structure, which isdifferent from film deposition by epitaxial growth. For example, evenwhen the top surface (formation surface) of the substrate 5120 has anamorphous structure (e.g., the top surface is formed of amorphoussilicon oxide), a CAAC-OS can be formed.

In addition, it is found that in formation of the CAAC-OS, the pellets5100 are arranged in accordance with the top surface shape of thesubstrate 5120 that is the formation surface even when the formationsurface has unevenness. For example, in the case where the top surfaceof the substrate 5120 is flat at the atomic level, the pellets 5100 arearranged so that flat planes parallel to the a-b plane face downwards.In the case where the thicknesses of the pellets 5100 are uniform, alayer with a uniform thickness, flatness, and high crystallinity isformed. By stacking n layers (n is a natural number), the CAAC-OS can beobtained.

In the case where the top surface of the substrate 5120 has unevenness,a CAAC-OS in which n layers (n is a natural number) in each of which thepellets 5100 are arranged along the unevenness are stacked is formed.Since the substrate 5120 has unevenness, a gap is easily generatedbetween the pellets 5100 in the CAAC-OS in some cases. Note that owingto intermolecular force, the pellets 5100 are arranged so that a gapbetween the pellets is as small as possible even on the unevennesssurface. Therefore, even when the formation surface has unevenness, aCAAC-OS with high crystallinity can be obtained.

As a result, laser crystallization is not needed for formation of aCAAC-OS, and a uniform film can be formed even over a large-sized glasssubstrate or the like.

Since a CAAC-OS is deposited in accordance with such a model, thesputtered particle preferably has a pellet shape with a small thickness.Note that when the sputtered particles have a dice shape with a largethickness, planes facing the substrate 5120 vary; thus, the thicknessesand orientations of the crystals cannot be uniform in some cases.

According to the deposition model described above, a CAAC-OS with highcrystallinity can be formed even on a formation surface with anamorphous structure.

FIG. 9C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 10, an optical system12 below the electron gun chamber 10, a sample chamber 14 below theoptical system 12, an optical system 16 below the sample chamber 14, anobservation chamber 20 below the optical system 16, a camera 18installed in the observation chamber 20, and a film chamber 22 below theobservation chamber 20. The camera 18 is provided to face toward theinside of the observation chamber 20. Note that the film chamber 22 isnot necessarily provided.

FIG. 9D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 9C. In thetransmission electron diffraction measurement apparatus, a substance 28which is positioned in the sample chamber 14 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 10 through the optical system 12. Electrons passing through thesubstance 28 enter a fluorescent plate 32 provided in the observationchamber 20 through the optical system 16. On the fluorescent plate 32, apattern corresponding to the intensity of entered electron appears,which allows measurement of a transmission electron diffraction pattern.

The camera 18 is installed so as to face the fluorescent plate 32 andcan take a picture of a pattern appearing in the fluorescent plate 32.An angle which is formed by a line passing through the center of a lensof the camera 18 and the top surface of the fluorescent plate 32, and aline which passes through the center of the lens of the camera 18 and isperpendicular to a floor is, for example, greater than or equal to 15°and less than or equal to 80°, greater than or equal to 30° and lessthan or equal to 75°, or greater than or equal to 45° and less than orequal to 70°. As the angle is reduced, distortion of the transmissionelectron diffraction pattern taken by the camera 18 becomes larger. Notethat if the angle is obtained in advance, the distortion of an obtainedtransmission electron diffraction pattern can be corrected. Note thatthe film chamber 22 may be provided with the camera 18. For example, thecamera 18 may be set in the film chamber 22 so as to be opposite to theincident direction of electrons 24 enter. In this case, a transmissionelectron diffraction pattern with less distortion can be taken from therear surface of the fluorescent plate 32.

A holder for fixing the substance 28 that is a sample is provided in thesample chamber 14. The holder transmits electrons passing through thesubstance 28. The holder may have, for example, a function of moving thesubstance 28 in the direction of the X, Y, and Z axes. The movementfunction of the holder may have an accuracy of moving the substance inthe range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10 nm to 100nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range is preferablydetermined to be an optimal range for the structure of the substance 28.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above is described.

For example, changes in the structure of a substance can be observed bychanging (scanning) the irradiation position of the electrons 24 thatare a nanobeam in the substance, as illustrated in FIG. 9D. At thistime, when the substance 28 is a CAAC-OS film, a diffraction patternshown in FIG. 9A can be observed. When the substance 28 is an nc-OSfilm, a diffraction pattern shown in FIG. 9B can be observed.

Even when the substance 28 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether or not the CAAC-OS film is favorable can bedetermined by the proportion of a region where a diffraction pattern ofthe CAAC-OS film is observed in a predetermined area (also referred toas proportion of CAAC). For example, in the case of a favorable CAAC-OSfilm, the proportion of CAAC is 60% or higher, preferably 80% or higher,further preferably 90% or higher, still preferably 95% or higher. Notethat a region where a diffraction pattern different from that of aCAAC-OS film is observed is referred to as the proportion of non-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedjust after deposition (represented as “as-sputtered”) and a top surfaceof a sample including a CAAC-OS subjected to heat treatment at 450° C.in an atmosphere containing oxygen. Here, the proportion of CAAC wasobtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/second and the obtaineddiffraction patterns were converted into still images every 0.5 seconds.Note that as an electron beam, a nano-electron beam with a probediameter of 1 nm was used. The above measurement was performed on sixsamples. The proportion of CAAC was calculated using the average valueof the six samples.

FIG. 10A shows the proportion of CAAC in each sample. The proportion ofCAAC of the CAAC-OS film obtained just after the deposition was 75.7%(the proportion of non-CAAC was 24.3%). The proportion of CAAC of theCAAC-OS film subjected to the heat treatment at 450° C. was 85.3% (theproportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained just after the deposition. That is, heattreatment at a high temperature (e.g., higher than or equal to 400° C.)reduces the proportion of non-CAAC (increases the proportion of CAAC).Furthermore, the above results also indicate that even when thetemperature of the heat treatment is lower than 500° C., the CAAC-OSfilm can have a high proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmare diffraction patterns similar to that of an nc-OS film. Furthermore,an amorphous oxide semiconductor film was not able to be observed in themeasurement region. Therefore, the above results suggest that the regionhaving a structure similar to that of an nc-OS film is rearranged by theheat treatment owing to the influence of the structure of the adjacentregion, whereby the region becomes CAAC.

FIGS. 10B and 10C are planar TEM images of the CAAC-OS film obtainedjust after the deposition and the CAAC-OS film subjected to the heattreatment at 450° C., respectively. Comparison between FIGS. 10B and 10Cshows that the CAAC-OS film subjected to the heat treatment at 450° C.has more uniform film quality. That is, the heat treatment at a hightemperature improves the film quality of the CAAC-OS film.

With such a measurement method, the structure of an oxide semiconductorlayer having a plurality of structures can be analyzed in some cases.

The transistor of one embodiment of the present invention can be formedusing an oxide semiconductor layer having any of the above structures.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3

In this embodiment, a semiconductor device which includes a transistorhaving a different structure from that of Embodiment 1 is described withreference to FIGS. 7A to 7C. The transistor described in this embodimentis different from those in Embodiment 1 in that a multilayer filmincluding a plurality of oxide semiconductor layers is provided. Here,details of the transistor are described using the semiconductor deviceillustrated in FIGS. 2A and 2B in Embodiment 1.

FIGS. 7A and 7B are a plan view and a cross-sectional view of atransistor 310 included in the semiconductor device of this embodiment.FIG. 7A is a plan view of the transistor 310, and FIG. 7B iscross-sectional views taken along dashed dotted lines A9-A10 and B9-B10in FIG. 7A. Note that the substrate 100 and some components (e.g., agate insulating layer) of the transistor 310 are not illustrated in FIG.7A for clarity. FIG. 7C shows a band diagram of a stacked-layerstructure included in the transistor 310.

The transistor 310 included in the semiconductor device illustrated inFIGS. 7A to 7C is different from the transistor 210 in FIGS. 2A and 2Bin that the oxide semiconductor layer provided between the gateinsulating layer 104 and the metal oxide layer 108 has a stacked-layerstructure including an oxide semiconductor layer 306 a and an oxidesemiconductor layer 306 b. The other components are similar to those inFIGS. 2A and 2B; thus, the above description can be referred to.

The oxide semiconductor layer 306 a and the oxide semiconductor layer306 b in the transistor 310 are each formed using a metal oxidecontaining at least In or Zn; as a typical example, an In—Ga oxide, anIn—Zn oxide, or an In-M-Zn oxide (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf)can be used. The energy of the bottom of the conduction band of theoxide semiconductor layer 306 b is closer to the vacuum level than thatof the oxide semiconductor layer 306 a; typically, an energy differencebetween the bottom of the conduction band of the oxide semiconductorlayer 306 b and the bottom of the conduction band of the oxidesemiconductor layer 306 a is greater than or equal to 0.05 eV, greaterthan or equal to 0.07 eV, greater than or equal to 0.1 eV, greater thanor equal to 0.15 eV, or greater than or equal to 0.5 eV, and less thanor equal to 2 eV or less than or equal to 1 eV. That is, the differencebetween the electron affinity of the oxide semiconductor layer 306 b andthe electron affinity of the oxide semiconductor layer 306 a is greaterthan or equal to 0.05 eV, greater than or equal to 0.07 eV, greater thanor equal to 0.1 eV, greater than or equal to 0.15 eV, or greater than orequal to 0.5 eV and also less than or equal to 2 eV, or less than orequal to 1 eV.

In such a structure, the oxide semiconductor layer 306 a serves as amain path of current and functions as a channel region when voltage isapplied to the transistor 310. In addition, since the oxidesemiconductor layer 306 b contains one or more kinds of metal elementsthat are contained in the oxide semiconductor layer 306 a where thechannel is formed, interface scattering is less likely to occur at theinterface between the oxide semiconductor layer 306 a and the oxidesemiconductor layer 306 b. Thus, the transistor can have highfield-effect mobility because the movement of carriers is not hinderedat the interface.

When the oxide semiconductor layer 306 b is formed of an In-M-Zn oxidein which the atomic ratio of the element M (M is Ti, Ga, Y, Zr, La, Ce,Nd, or Hf) is higher than that of In, the energy gap of the oxidesemiconductor layer 306 b can be large and the electron affinity can besmall. Therefore, a difference in electron affinity between the oxidesemiconductor layer 306 a and the oxide semiconductor layer 306 b may becontrolled by the proportion of the element M. Furthermore, oxygenvacancy is less likely to be generated in the oxide semiconductor layerin which the atomic ratio of Ti, Ga, Y, Zr, La, Ce, Nd, or Hf is higherthan that of In because Ti, Ga, Y, Zr, La, Ce, Nd, and Hf each are ametal element that is strongly bonded to oxygen.

In the case where the oxide semiconductor layer 306 b is formed of anIn-M-Zn oxide, when Zn and O are eliminated from consideration, theatomic percentage of In and the atomic percentage of M are preferablyless than 50 atomic % and greater than or equal to 50 atomic %,respectively, more preferably less than 25 atomic % and greater than orequal to 75 atomic %, respectively.

Furthermore, in the case where each of the oxide semiconductor layer 306a and the oxide semiconductor layer 306 b is formed of In-M-Zn oxide (Mis Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), the atomic percent of M (M is Ti,Ga, Y, Zr, La, Ce, Nd, or Hf) in the oxide semiconductor layer 306 b ishigher than that in the oxide semiconductor layer 306 a. Typically, theatomic percentage of M in the oxide semiconductor layer 306 b is 1.5 ormore times, twice or more, or three or more times as high as that in theoxide semiconductor layer 306 a.

Furthermore, in the case where each of the oxide semiconductor layer 306a and the oxide semiconductor layer 306 b is formed of an In-M-Zn oxide(M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), when the oxide semiconductorlayer 306 a has an atomic ratio of In:M:Zn=x₁:y₁:z₁ and the oxidesemiconductor layer 306 b has an atomic ratio of In:M:Zn=x₂:y₂:z₂, y₂/x₂is higher than y₁/x₁. It is preferable that y₂/x₂ be 1.5 or more timesas high as y₁/x₁. It is further preferable that y₂/x₁ be twice or moreas high as y₁/x₁. It is still further preferable that y₂/x₂ be three ormore times as high as y₁/x₁. In this case, it is preferable that in theoxide semiconductor layer, y₁ be higher than or equal to x₁ because atransistor including the oxide semiconductor layer can have stableelectric characteristics. However, when y, is higher than or equal tothree times x₁, the field-effect mobility of the transistor includingthe oxide semiconductor layer is reduced. Thus, it is preferable that y₁be lower than three times x₁. The composition of the oxide semiconductorlayer can be measured by ICP-MS. For example, a metal oxide film that isobtained under conditions where a target with In₂O₃:Ga₂O₃:ZnO=1:1:1(In:Ga:Zn=1:1:0.5) is used and the flow ratio of an argon gas in asputtering method is 40 sccm is InGa_(0.95)Zn_(0.41)O_(3.33).Furthermore, the composition can be quantified using a Rutherfordbackscattering spectrometry (RBS) instead of ICP-MS.

In the case where the oxide semiconductor layer 306 a is formed of anIn-M-Zn oxide (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) and a targethaving the atomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is usedfor depositing the oxide semiconductor layer 306 a, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6, andz₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₁/y₁ is greater than or equal to 1 and less thanor equal to 6, CAAC-OS film is easily formed as the oxide semiconductorlayer 306 a. Typical examples of the atomic ratio of the metal elementsof the target include In:M:Zn=1:1:1 and In:M:Zn=3:1:2.

Note that in the case where a target with In:M:Zn=1:1:z₁₀ is used fordepositing the oxide semiconductor layer 306 a, z₁₀ is preferablygreater than or equal to 1 and less than or equal to 1.4, furtherpreferably greater than or equal to 1 and less than or equal to 1.3.This is because, for example, when In:M:Zn is 1:1:1.5, the targetbecomes opaque, and sputtering deposition with a DC power source or anAC power source might become difficult. Such a target is applicable todeposition using an RF power source; however, in consideration ofproductivity of the semiconductor device, it is preferable to use atarget which is applicable to a sputtering deposition using a DC powersource or an AC power source.

In the case where the oxide semiconductor layer 306 b is formed of anIn-M-Zn oxide (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) and a targethaving an atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used fordepositing the oxide semiconductor layer 306 b, x ₂/y₂ is preferablyless than x₁/y₁, and z₂/y₂ is preferably greater than or equal to ⅓ andless than or equal to 6, further preferably greater than or equal to 1and less than or equal to 6. When the atomic ratio of M with respect toindium is high, the energy gap of the oxide semiconductor layer 306 bcan be large and the electron affinity thereof can be small; therefore,y₂/x₂ is preferably higher than or equal to 3 or higher than or equal to4. Typical examples of the atomic ratio of the metal elements of thetarget include In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:5,In:M:Zn=1:3:6, In:M:Zn=1:4:2, In:M:Zn=1:4:4, and In:M:Zn=1:4:5.

For example, in the case where a target with In:M:Zn=1:3:z₂₀ is used asthe target for depositing the oxide semiconductor layer 306 b, z₂₀ ispreferably greater than or equal to 2 and less than or equal to 5.Alternatively, in the case where a target having an atomic ratio ofIn:M:Zn=1:4:z₃₀ is used for depositing the oxide semiconductor layer 306b, z₃₀ is preferably greater than or equal to 2 and less than or equalto 5.

Note that in each of the oxide semiconductor layer 306 a and the oxidesemiconductor layer 306 b, the proportions of atoms in the atomic ratiovaries within a range of ±40% as an error.

It is preferable that the oxide semiconductor layer 306 a and the oxidesemiconductor layer 306 b have crystal parts, further preferably, havethe same crystal structures. This is because when the oxidesemiconductor layer 306 a and the oxide semiconductor layer 306 b havedifferent crystal structures, the interface between the layers becomes ahetero crystalline structure part and a defect might be generatedtherein. The hetero crystalline structure part can be regarded as, forexample, a grain boundary.

As the oxide semiconductor layer 306 a, a CAAC-OS film that is an oxidesemiconductor layer having a low impurity concentration and low densityof defect states (a small amount of oxygen vacancy) is preferably used.The state in which impurity concentration is low and density of defectstates is low is referred to as highly purified intrinsic or highlypurified substantially intrinsic. A highly purified intrinsic or highlypurified substantially intrinsic oxide semiconductor layer has fewcarrier generation sources, and thus has a low carrier density. Thus, atransistor using the oxide semiconductor layer as a channel rarely haselectrical characteristics in which a threshold voltage is negative(also referred to as normally-on). A highly purified intrinsic or highlypurified substantially intrinsic oxide semiconductor layer has fewcarrier traps. Thus, the transistor including the oxide semiconductorlayer in the channel has a small variation in electrical characteristicsand high reliability. With the use of the CAAC-OS film in a transistor,variation in the electrical characteristics of the transistor due toirradiation with visible light or ultraviolet light is small.

Note that it is preferable that a target with In:M:Zn=1:1:1.2 be usedfor depositing the oxide semiconductor layer 306 a because a spinelstructure is less likely to be formed in the deposited oxidesemiconductor layer 306 a, so that the proportion of CAAC can beincreased.

Furthermore, in the case where the oxide semiconductor layer 306 a is aCAAC-OS film and the oxide semiconductor layer 306 b in contact with theoxide semiconductor layer 306 a has a different crystal structure, agrain boundary is formed at the interface between the two layers and adefect might be formed in the film; therefore, it is preferable to use aCAAC-OS film also for the oxide semiconductor layer 306 b.

Meanwhile, in the case where, for example, an In—Ga oxide layer isformed as the metal oxide layer 108, which functions as a barrier layerfor preventing mixing of impurities to the oxide semiconductor layers306 a and 306 b, the In—Ga oxide layer can have an amorphous structure,a crystalline structure similar to that of an nc-OS film, or amonoclinic structure; however, it is difficult for the In—Ga oxide layerto have a crystalline structure similar to that of a CAAC-OS film.Therefore, when the oxide semiconductor layer 306 a where the channel isformed is in contact with the metal oxide layer 108, a hetero structuremight be formed at the interface between the two layers. In thetransistor 310 described in this embodiment, since the oxidesemiconductor layer 306 b is provided between the metal oxide layer 108and the oxide semiconductor layer 306 a where the channel is formed, theregion in contact with the hetero structure can be apart from the oxidesemiconductor layer 306 a where carriers flow. However, the oxidesemiconductor layer 306 b may have a spinel structure therein. This isbecause the metal oxide layer 108 can prevent the constituent elementsof the pair of electrode layers 110 a and 110 b from diffusing into theoxide semiconductor layer 306 b; therefore, even when the oxidesemiconductor layer 306 b has a spinel structure, diffusion of a metalelement such as copper which is derived from the spinel structure, tothe channel can be prevented.

FIG. 7C is an example of a band structure in the thickness direction ofthe stacked-layer structure including the gate insulating layer 104, theoxide semiconductor layer 306 a, the oxide semiconductor layer 306 b,the metal oxide layer 108, and the oxide insulating layer 112. For easyunderstanding, the energy (Ec) of the bottom of the conduction band ofeach of the gate insulating layer 104, the oxide semiconductor layer 306a, the oxide semiconductor layer 306 b, the metal oxide layer 108, andthe oxide insulating layer 112 is shown in the band structure.

As shown in FIG. 7C, there is no energy barrier between the oxidesemiconductor layers 306 a and 306 b, and the energy of the bottom ofthe conduction band is changed smoothly (such a state is also referredto as a continuous junction). In other words, the energy of the bottomof the conduction band is continuously changed. To obtain such a bandstructure, it is preferable that an impurity which forms a defect levelsuch as a trap center or a recombination center does not exist at theinterface between the oxide semiconductor layer 306 a and the oxidesemiconductor layer 306 b. This is because if an impurity exists betweenthe stacked oxide semiconductor layers, a continuity of the energy bandis damaged, and the carrier is captured or recombined at the interfaceand then disappears.

To form a continuous junction between the oxide semiconductor layers 306a and 306 b, it is necessary to form films continuously without beingexposed to air, with use of a multi-chamber deposition apparatus(sputtering apparatus) including a load lock chamber.

With the structure of FIG. 7C, the oxide semiconductor layer 306 aserves as a well, and a channel region is formed in the oxidesemiconductor layer 306 a in the transistor with the stacked layerstructure.

Although trap levels due to impurities or defects might be formed in thevicinity of the interface between the metal oxide layer 108 and theoxide insulating layer 112 as shown in FIG. 8A, the oxide semiconductorlayers 306 a and 306 b can be distanced from the trap levels owing tothe existence of the metal oxide layer 108. Furthermore, even whendefects due to the hetero crystalline structure exist between the metaloxide layer 108 and the oxide semiconductor layer 306 b, the oxidesemiconductor layer 306 b can reduce the influence of the defects uponthe oxide semiconductor layer 306 a. Here, in the case where an energydifference between the bottom of the conduction band of the oxidesemiconductor layer 306 a and that of the oxide semiconductor layer 306b is small, electrons in the oxide semiconductor layer 306 a might reachthe trap level by passing through the energy difference. Since theelectron is trapped at the trap level, a negative fixed charge isgenerated, causing the threshold voltage of the transistor to be shiftedin the positive direction. Thus, it is preferable that the energydifference between the bottom of the conduction band of the oxidesemiconductor layer 306 a and that of the oxide semiconductor layer 306b be 0.1 eV or more, preferably 0.15 eV or more because a change in thethreshold voltage of the transistor is reduced and stable electricalcharacteristics are obtained.

It is preferable that the difference in energy of the bottom of theconduction band between the oxide semiconductor layer 306 a and theoxide semiconductor layer 306 b be greater than or equal to 0.1 eV,further preferably greater than or equal to 0.15 eV because the traplevel existing in the vicinity of the interface between the metal oxidelayer 108 and the oxide insulating layer 112 can be prevented fromaffecting the oxide semiconductor layer 306 b and the oxidesemiconductor layer 306 a in contact with the oxide semiconductor layer306 b.

Note that the structure of the transistor having the stacked-layerstructure that is described in this embodiment is not limited to that ofFIGS. 7A to 7C. For example, like in a transistor 300 illustrated inFIG. 21A, the oxide semiconductor layer provided between the gateinsulating layer 104 and the metal oxide layer 108 may have astacked-layer structure including an oxide semiconductor layer 316 a andan oxide semiconductor layer 316 b in the structure of the transistor200 described in Embodiment 1. Note that FIG. 21A illustrates a crosssection of the transistor 300 in the channel length direction and across section of a connection portion between an electrode layer 202 bwhich is formed in the same layer as a gate electrode layer 202 a and anelectrode layer 110 c which is formed in the same layer as the pair ofelectrode layers 110 a and 110 b.

In the transistor 300 in FIG. 21A, the gate electrode layer 202 a andthe electrode layer 202 b that is formed in the same layer as the gateelectrode layer 202 a have a stacked-layer structure including firstconductive layers 101 a and 101 b and a stacked-layer structureincluding second conductive layers 103 a and 103 b, respectively. Amaterial similar to that of the first conductive layer 109 a and 109 bof the pair of electrode layers 110 a and 110 b can be used for thefirst conductive layers 101 a and 101 b. A material similar to that ofthe second conductive layers 111 a and 111 b of the pair of electrodelayers 110 a and 110 b can be used for the second conductive layers 103a and 103 b.

When the gate electrode layer 202 a and the electrode layer 202 b areformed to contain a low-resistance material such as copper, aluminum,gold, or silver, it is possible to manufacture a semiconductor devicewith reduced wiring delay even in the case of using a large-sizedsubstrate as the substrate 100. Note that in the case where electrodelayers containing any of the above low-resistance materials are formedas the gate electrode layer 202 a and the electrode layer 202 b, it ispreferable that the gate insulating layer 104 have a stacked-layerstructure including a nitride insulating layer 104 a and an oxideinsulating layer 104 b and that the oxide insulating layer 104 b be incontact with the oxide semiconductor layer 316 a. The nitride insulatinglayer 104 a included in the gate insulating layer 104 can be used as abarrier layer for preventing diffusion of the low-resistance material.The oxide insulating layer 104 b prevents diffusion of nitrogen from thenitride insulating layer 104 a to the oxide semiconductor layers 316 aand 316 b and functions as a supply source of oxygen for the oxidesemiconductor layers 316 a and 316 b.

The structure of the oxide semiconductor layer 316 a included in thetransistor 300 can be the same as that of the oxide semiconductor layer306 a of the transistor 310; therefore, the above description can bereferred to. The structure of the oxide semiconductor layer 316 b can bethe same as that of the oxide semiconductor layer 306 b of thetransistor 310; therefore, the above description can be referred to.Therefore, in the band structure in the thickness direction of thestacked-layer structure in the transistor 300, which includes the gateinsulating layer 104, the oxide semiconductor layer 316 a, the oxidesemiconductor layer 316 b, the metal oxide layer 108, and the oxideinsulating layer 112, as shown in FIG. 21B, the oxide semiconductorlayer 316 a serves as a well; thus, the channel region is formed in theoxide semiconductor layer 316 a in the transistor including thestacked-layer structure.

Note that the connection between the electrode layer 202 b and theelectrode layer 110 c in the transistor 300 is formed in such a mannerthat a metal oxide film and an oxide semiconductor film are processedinto an island shape, and an opening portion is formed in the gateinsulating layer 104 to expose the electrode layer 202 b. After that, aconductive film to be the pair of electrode layers 110 a and 110 b andthe electrode layer 110 c is formed and processed, whereby the electrodelayer 202 b and the electrode layer 110 c can be connected to eachother.

The structure described in this embodiment makes it possible to obtain ahighly reliable transistor in which the impurity concentration of anoxide semiconductor layer including the channel formation region isreduced. Furthermore, the channel is less likely to be influenced by theinterface state in the structure, so that a reduction in on-statecurrent due to the interface state is less likely to occur. Accordingly,the transistor can have high on-state current and small S-value. Inaddition, a change in electrical characteristics due to the interfacestate is less likely to occur in the transistor, whereby the transistorhas high reliability.

Note that the structure and method described in this embodiment can beimplemented by being combined as appropriate with any of the otherstructures methods described in the other embodiments.

Embodiment 4

In this embodiment, a structural example of a display panel as asemiconductor device of one embodiment of the present invention isdescribed.

<Display Panel>

A display panel including a semiconductor device such as any of theabove-described transistors is described below.

FIG. 18A is a top view of the display panel of one embodiment of thepresent invention. FIG. 18B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display panel of one embodiment of the presentinvention. FIG. 18C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display panel of one embodiment of the present invention.

As the transistor to be disposed in the pixel portion, the transistordescribed in Embodiment 1 or 3 can be used. Further, the transistor caneasily be an n-channel transistor, and thus, part of a driver circuitthat can be formed using an n-channel transistor in the driver circuitis formed over the same substrate as the transistor of the pixelportion. With the use of the transistor described in Embodiment 1 or 3for the pixel portion or the driver circuit in this manner, a highlyreliable display device can be provided.

FIG. 18A is an example of a block diagram of an active matrix displaydevice. A pixel portion 701, a first scan line driver circuit 702, asecond scan line driver circuit 703, and a signal line driver circuit704 are formed over a substrate 700 of the display device. In the pixelportion 701, a plurality of signal lines extended from the signal linedriver circuit 704 is arranged and a plurality of scan lines extendedfrom the first scan line driver circuit 702 and the second scan linedriver circuit 703 is arranged. Note that pixels each including adisplay element are provided in matrix in respective regions in each ofwhich the scan line and the signal line intersect with each other. Thesubstrate 700 of the display device is connected to a timing controlcircuit (also referred to as a controller or a controller IC) through aconnection portion such as a flexible printed circuit (FPC).

In FIG. 18A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the same substrate 700 as the pixel portion 701.Accordingly, the number of components that are provided outside, such asa driver circuit, can be reduced, so that a reduction in cost can beachieved. Furthermore, if the driver circuit is provided outside thesubstrate 700, wirings would need to be extended and the number ofconnections of wirings would be increased, but by providing the drivercircuit over the substrate 700, the number of connections of the wiringscan be reduced. Consequently, an improvement in reliability or yield canbe achieved.

[Liquid Crystal Panel]

FIG. 18B illustrates an example of a circuit configuration of a pixel ina liquid crystal panel as one mode of the display panel. Here, a pixelcircuit which is applicable to a pixel of a VA liquid crystal displaypanel is illustrated.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A gate wiring 712 of a transistor 716 and a gate wiring 713 of atransistor 717 are separated so that different gate signals can besupplied thereto. In contrast, a source or drain electrode layer 714that functions as a data line is shared by the transistors 716 and 717.The transistor described in Embodiment 3 can be used as appropriate aseach of the transistors 716 and 717. In the above manner, a highlyreliable liquid crystal display panel can be provided.

The shapes of a first pixel electrode layer electrically connected tothe transistor 716 and a second pixel electrode layer electricallyconnected to the transistor 717 are described. The first pixel electrodelayer and the second pixel electrode layer are separated by a slit. Thefirst pixel electrode layer has a V shape and the second pixel electrodelayer is provided so as to surround the first pixel electrode layer.

A gate electrode of the transistor 716 is connected to the gate wiring712, and a gate electrode of the transistor 717 is connected to the gatewiring 713. When different gate signals are supplied to the gate wiring712 and the gate wiring 713, operation timings of the transistor 716 andthe transistor 717 can be varied. As a result, alignment of liquidcrystals can be controlled.

In addition, a storage capacitor may be formed using a capacitor wiring710, a gate insulating layer functioning as a dielectric, and acapacitor electrode electrically connected to the first pixel electrodelayer or the second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of one embodiment of the present invention isnot limited to that shown in FIG. 18B. For example, a switch, aresistor, a capacitor, a transistor, a sensor, or a logic circuit may beadded to the pixel illustrated in FIG. 18B.

[Organic EL Panel]

As another mode of the display panel, an example of a circuitconfiguration of a pixel of an organic EL panel is shown in FIG. 18C.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.Then, recombination of the electrons and holes makes the light-emittingorganic compound to form an excited state and to emit light when itreturns from the excited state to a ground state. Based on such amechanism, such a light-emitting element is referred to as acurrent-excitation type light-emitting element.

FIG. 18C illustrates an applicable example of a pixel circuit. In thisexample, one pixel includes two n-channel transistors. Note that themetal oxide film of one embodiment of the present invention can be usedfor channel formation regions of the n-channel transistors. Furthermore,digital time grayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line provided over the same substrate.

As the switching transistor 721 and the driver transistor 722, thetransistor described in Embodiment 3 can be used as appropriate. In thismanner, a highly reliable organic EL display panel can be provided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least forward thresholdvoltage.

Note that gate capacitance of the driver transistor 722 may be used as asubstitute for the capacitor 723, so that the capacitor 723 can beomitted. The gate capacitance of the driver transistor 722 may be formedbetween the channel formation region and the gate electrode layer.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order that the driver transistor 722 isoperated in a linear region, voltage higher than the voltage of thepower supply line 727 is applied to the gate electrode layer of thedriver transistor 722. Note that voltage higher than or equal to voltagewhich is the sum of power supply line voltage and the threshold voltageV_(th) of the driver transistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage greaterthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage Vth of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order that the driver transistor722 is operated in a saturation region, the potential of the powersupply line 727 is set higher than the gate potential of the drivertransistor 722. When an analog video signal is used, it is possible tosupply current to the light-emitting element 724 in accordance with thevideo signal and perform analog grayscale driving.

Note that the configuration of the pixel circuit is not limited to thatshown in FIG. 18C. For example, a switch, a resistor, a capacitor, asensor, a transistor, a logic circuit, or the like may be added to thepixel circuit illustrated in FIG. 18C.

In the case where the transistor described in Embodiment 1 or 3 is usedfor the circuit shown in FIGS. 18A to 18C, the source electrode layer iselectrically connected to the low potential side and the drain electrodelayer is electrically connected to the high potential side.

For example, in this specification and the like, a display element, adisplay device, which is a device including a display element, alight-emitting element, and a light-emitting device, which is a deviceincluding a light-emitting element, can employ various modes or caninclude various elements. Examples of a display element, a displaydevice, a light-emitting element, or a light-emitting device include adisplay medium whose contrast, luminance, reflectance, transmittance, orthe like is changed by electromagnetic action, such as anelectroluminescence (EL) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, a liquid crystal element, electronic ink, anelectrophoretic element, a grating light valve (GLV), a plasma displaypanel (PDP), a micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), interferometricmodulator display (IMOD) element, an electrowetting element, apiezoelectric ceramic display, or a carbon nanotube. Note that examplesof display devices having EL elements include an EL display. Examples ofdisplay devices including electron emitters are a field emission display(FED) and an SED-type flat panel display (SED: surface-conductionelectron-emitter display). Examples of display devices including liquidcrystal elements include a liquid crystal display (e.g., a transmissiveliquid crystal display, a transflective liquid crystal display, areflective liquid crystal display, a direct-view liquid crystal display,or a projection liquid crystal display). An example of a display deviceincluding electronic ink or electrophoretic elements is electronicpaper.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 5

In this embodiment, a display module and electronic appliances that canbe formed using a semiconductor device of one embodiment of the presentinvention are described.

In a display module 8000 illustrated in FIG. 19, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.Note that the backlight unit 8007, the battery 8011, the touch panel8004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be used overlapping with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel. An electrode fora touch sensor may be provided in each pixel of the display panel 8006so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 20A to 20D are external views of electronic appliances eachincluding the semiconductor device of one embodiment of the presentinvention.

Examples of electronic appliances are a television set (also referred toas a television or a television receiver), a monitor of a computer orthe like, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 20A illustrates a portable information terminal including a mainbody 1001, a housing 1002, display portions 1003 a and 1003 b, and thelike. The display portion 1003 b is a touch panel. By touching akeyboard button 1004 displayed on the display portion 1003 b, a screencan be operated, and text can be input. It is needless to say that thedisplay portion 1003 a may be a touch panel. A liquid crystal panel oran organic light-emitting panel is fabricated using any of thetransistors described in the above embodiments as a switching elementand used in the display portion 1003 a or 1003 b, whereby a highlyreliable portable information terminal can be provided.

The portable information terminal illustrated in FIG. 20A can have afunction of displaying various kinds of information (e.g., a stillimage, a moving image, and a text image); a function of displaying acalendar, the date, the time, and the like on the display portion; afunction of operating or editing the information displayed on thedisplay portion; a function of controlling processing by various kindsof software (programs); and the like. Furthermore, an externalconnection terminal (an earphone terminal, a USB terminal, or the like),a recording medium insertion portion, and the like may be provided onthe back surface or the side surface of the housing.

The portable information terminal illustrated in FIG. 20A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an e-bookserver.

FIG. 20B illustrates a portable music player including, in a main body1021, a display portion 1023, a fixing portion 1022 with which theportable music player can be worn on the ear, a speaker, an operationbutton 1024, an external memory slot 1025, and the like. A liquidcrystal panel or an organic light-emitting panel is fabricated using anyof the transistors described in the above embodiments as a switchingelement and used in the display portion 1023, whereby a highly reliableportable music player can be provided.

Furthermore, when the portable music player illustrated in FIG. 20B hasan antenna, a microphone function, or a wireless communication functionand is used with a mobile phone, a user can talk on the phone wirelesslyin a hands-free way while driving a car or the like.

FIG. 20C illustrates a mobile phone including two housings, a housing1030 and a housing 1031. The housing 1031 includes a display panel 1032,a speaker 1033, a microphone 1034, a pointing device 1036, a camera1037, an external connection terminal 1038, and the like. The housing1030 is provided with a solar cell 1040 for charging the mobile phone,an external memory slot 1041, and the like. In addition, an antenna isincorporated in the housing 1031. Any of the transistors described inthe above embodiments is used in the display panel 1032, whereby ahighly reliable mobile phone can be provided.

Furthermore, the display panel 1032 includes a touch panel. A pluralityof operation keys 1035 which are displayed as images are indicated bydotted lines in FIG. 20C. Note that a boosting circuit by which avoltage output from the solar cell 1040 is increased to be sufficientlyhigh for each circuit is also included.

In the display panel 1032, the direction of display is changed asappropriate depending on the application mode. Furthermore, the mobilephone is provided with the camera 1037 on the same surface as thedisplay panel 1032, and thus it can be used as a video phone. Thespeaker 1033 and the microphone 1034 can be used for videophone calls,recording, and playing sound, etc. as well as voice calls. Moreover, thehousings 1030 and 1031 in a state where they are developed asillustrated in FIG. 20C can shift, by sliding, to a state where oneoverlaps with the other. Therefore, the size of the mobile phone can bereduced, which makes the mobile phone suitable for being carried around.

The external connection terminal 1038 can be connected to an AC adaptorand a variety of cables such as a USB cable, whereby charging and datacommunication with a personal computer or the like are possible. Inaddition, by inserting a recording medium into the external memory slot1041, a larger amount of data can be stored and moved.

In addition to the above functions, an infrared communication function,a television reception function, or the like may be provided.

FIG. 20D illustrates an example of a television set. In a television set1050, a display portion 1053 is incorporated in a housing 1051. Imagescan be displayed on the display portion 1053. Moreover, a CPU isincorporated in a stand 1055 for supporting the housing 1051. Any of thetransistors described in the above embodiments is used in the displayportion 1053 and the CPU, whereby the television set 1050 can be highlyreliable.

The television set 1050 can be operated with an operation switch of thehousing 1051 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 1050 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

Furthermore, the television set 1050 is provided with an externalconnection terminal 1054, a storage medium recording and reproducingportion 1052, and an external memory slot. The external connectionterminal 1054 can be connected to various types of cables such as a USBcable, and data communication with a personal computer or the like ispossible. A disk storage medium is inserted into the storage mediumrecording and reproducing portion 1052, and reading data stored in thestorage medium and writing data to the storage medium can be performed.In addition, an image, a video, or the like stored as data in anexternal memory 1056 inserted into the external memory slot can bedisplayed on the display portion 1053.

Further, in the case where the off-state leakage current of thetransistor described in the above embodiments is extremely small, whenthe transistor is used in the external memory 1056 or the CPU, thetelevision set 1050 can have high reliability and sufficiently reducedpower consumption.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Example 1

In this example, transistors of one embodiment of the present inventionwere formed and their initial characteristics were measured.Furthermore, a band diagram of an oxide semiconductor layer and a metaloxide layer included in each transistor was measured. In addition,diffusion of copper in the metal oxide layer included in each transistorwas evaluated. Results thereof are described.

First, a method for forming the transistors used in this example isdescribed below. In this example, transistors having a structure similarto that of the transistor 200 illustrated in FIGS. 1A and 1B wereformed.

(Sample A1)

A method for forming Sample A1 is described.

A glass substrate was used as the substrate 100, and a 150-nm-thicktungsten film was deposited as a conductive film over the substrate 100by a sputtering method. Next, the conductive film was selectivelyprocessed using a mask formed by a photolithography method to form thegate electrode layer 102.

Then, the gate insulating layer 104 was formed over the substrate 100and the gate electrode layer 102. Here, as the gate insulating layer104, a 400-nm-thick silicon nitride film and a 50-nm-thick siliconoxynitride film were deposited by a CVD method.

Next, as an oxide semiconductor film, a 35-nm-thick In—Ga—Zn oxide film(hereinafter also referred to as IGZO(1:1:1)) was deposited over thegate insulating layer 104 by a sputtering method using an oxide targethaving an atomic ratio of In:Ga:Zn=1:1:1. Deposition conditions were asfollows; an atmosphere of argon and oxygen (argon:oxygen=20 sccm:10sccm), a pressure of 0.4 Pa, a power (DC) of 200 kW, and a substratetemperature of 300° C. After the oxide semiconductor film was formed, ametal oxide film was successively formed without exposure to the air. Asthe metal oxide film, a 20-nm-thick In—Ga oxide film (hereinafter alsoreferred to as IGO(1:1)) was deposited by a sputtering method using anoxide target having an atomic ratio of In:Ga=7:93. Deposition conditionswere as follows; an atmosphere of argon and oxygen (argon:oxygen=20sccm:10 sccm), a pressure of 0.4 Pa, a power (DC) of 200 kW, and asubstrate temperature of 300° C.

After the heat treatment, the oxide semiconductor film and the metaloxide film were processed into an island shape using a mask formed by aphotolithography method to form the oxide semiconductor layer 106 andthe metal oxide layer 108.

Next, heat treatment was performed at 450° C. for one hour in a nitrogenatmosphere, and then heat treatment was performed at 450° C. for onehour in a mixed atmosphere containing oxygen and nitrogen in the sametreatment chamber.

A 30-nm-thick tungsten film and a 200-nm-thick copper film weredeposited as a conductive film over the oxide semiconductor layer 106and metal oxide layer 108 which had island shapes.

Then, the tungsten film and the copper film were selectively etchedusing a mask formed by a photolithography method to form the pair ofelectrode layers 110 a and 110 b.

Next, a 50-nm-thick silicon oxynitride film was deposited as the oxideinsulating layer 112 over the gate insulating layer 104, the metal oxidelayer 108, and the pair of electrode layers 110 a and 110 b by a CVDmethod. Subsequently, a 400-nm-thick silicon oxynitride film wassuccessively deposited as the oxide insulating layer 114 by a CVD methodwithout exposure to the air.

After that, heat treatment was performed at 350° C. for one hour in amixed atmosphere containing oxygen and nitrogen.

Next, a 100-nm-thick silicon nitride film was deposited as the nitrideinsulating layer 116 over the oxide insulating layer 114 by a CVDmethod.

Then, although not illustrated, part of each of the oxide insulatinglayer 112, the oxide insulating layer 114, and the nitride insulatinglayer 116 was etched using a mask formed by a photolithography method toform an opening portion where one of the pair of electrode layers 110 aand 110 b was exposed.

Subsequently, a 100-nm-thick indium oxide-tin oxide compound (ITO-SiO₂)film containing silicon oxide was formed as a conductive film over thenitride insulating layer 116 by a sputtering method. Then, part of theconductive film was etched using a mask formed by a photolithographymethod to form a conductive layer in contact with one of the pair ofelectrode layers 110 a and 110 b. After that, heat treatment wasperformed at 250° C. for one hour in a nitrogen atmosphere.

Next, a 1.6-μm-thick polyimide layer was formed as a planarization layer(not illustrated) over the nitride insulating layer 116 and theconductive layer. Here, after a composition was applied to the nitrideinsulating layer 116, light exposure and development were performed, andheat treatment was performed at 300° C. for one hour in an atmospherecontaining nitrogen, whereby to form the planarization layer having anopening portion where part of the pair of electrode layers 110 a and 110b was exposed.

Through the above process, Sample A1 was formed.

(Sample A2)

Sample A2, which is a comparative example, was formed under the sameformation conditions as those of Sample A1 to have the same structure asSample A1 except that the metal oxide layer 108 is not provided.

(Sample A3)

Sample A3, which is a comparative example, was formed under the sameformation conditions as Sample A1 to have the same structure as SampleA1 except that an oxide semiconductor layer is provided instead of themetal oxide layer 108. Specifically, a sample in which an oxidesemiconductor film to be the oxide semiconductor layer was depositedunder the following conditions was formed as Sample A3.

As the oxide semiconductor film, a 20-nm-thick In—Ga—Zn oxide film(hereinafter also referred to as IGZO(1:3:6)) was formed by a sputteringmethod using an oxide target with an atomic ratio of In:Ga:Zn=1:3:6.Deposition conditions were as follows; an atmosphere of argon and oxygen(argon:oxygen=20 sccm:10 sccm), a pressure of 0.4 Pa, a power (DC) of200 kW, and a substrate temperature of 200° C.

(Sample A4)

Sample A4, which is a comparative example, was formed under the sameformation conditions as Sample A1 to have the same structure as SampleA1 except that an oxide semiconductor layer is provided instead of themetal oxide layer 108. Specifically, a sample in which an oxidesemiconductor film to be the oxide semiconductor layer was depositedunder the following conditions was formed as Sample A4.

As the oxide semiconductor film, a 20-nm-thick In—Ga—Zn oxide film (alsoreferred to as IGZO(1:6:4)) was formed by a sputtering method using anoxide target with an atomic ratio of In:Ga:Zn=1:6:4. Depositionconditions were as follows; an atmosphere of argon and oxygen(argon:oxygen=20 sccm:10 sccm), a pressure of 0.4 Pa, a power (DC) of200 kW, and a substrate temperature of 200° C.

(Sample A5)

Sample A5, which is a comparative example, was formed under the sameformation conditions as those of Sample A1 to have the same structure asSample A1 except that a metal oxide layer in which the atomic ratio ofindium to gallium is different from that in the metal oxide layer 108 isprovided instead of the metal oxide layer 108. Specifically, a sample inwhich a metal oxide film to be the metal oxide layer was deposited underthe following conditions was formed as Sample A5.

As the metal oxide layer, a 20-nm-thick In—Ga oxide film was depositedby a sputtering method using an oxide target having an atomic ratio ofIn:Ga=2:1 (such a metal oxide layer is also referred to as IGO(2:1)).Deposition conditions were as follows; an atmosphere of argon and oxygen(argon:oxygen=20 sccm:10 sccm), a pressure of 0.4 Pa, a power (DC) of200 kW, and a substrate temperature of 300° C.

(Vg-Id Characteristics)

Next, Vg-Id characteristics of the transistors included in Sample A1 toSample A5 were measured. Here, changes in characteristics of currentflowing between a source electrode layer and a drain electrode layer(hereinafter referred to as drain current: Id), that is, Vg-Idcharacteristics were measured under the following conditions; thesubstrate temperature was 25° C., the potential difference between thesource electrode layer and the drain electrode layer (hereinafterreferred to as drain voltage: Vd) was 1 V or 10 V, and the potentialdifference between the source electrode layer and the gate electrodelayer (hereinafter referred to as gate voltage: Vg) was changed from −20V to 20 V. In each sample, the channel length L of the transistor was 6μm and the channel width W thereof was 50 μm. Furthermore, each sampleincludes four transistors.

FIG. 11A shows Vg-Id characteristics of the transistors included inSample A1. FIG. 12A shows Vg-Id characteristics of the transistorsincluded in Sample A2. FIG. 13A shows Vg-Id characteristics of thetransistors included in Sample A3. FIG. 14A shows Vg-Id characteristicsof the transistors included in Sample A4. FIG. 15A shows Vg-Idcharacteristics of the transistors included in Sample A5. In each ofFIG. 11A, FIG. 12A, FIG. 13A, FIG. 14A, and FIG. 15A, the horizontalaxis represents gate voltage Vg, the first vertical axis representsdrain current Id, and the second vertical axis represent field-effectmobility. Here, to show field-effect mobility in a saturation region,field-effect mobility calculated when Vd=10 V is shown.

FIG. 11A shows that the transistors of Sample A1 have high on-statecurrent and excellent Vg-Id characteristics.

Meanwhile the Vg-Id characteristics in FIG. 12A reveal that on-statecurrent is reduced in the transistors of Sample A2. A possible cause ofthe reduction in on-state current is trapping of a conduction electrondue to a shallow trap level in an oxide semiconductor layer. The shallowtrap level is formed owing to Cu which has been included in the pair ofelectrode layers 110 a and 110 b and then moved to the surface of theoxide semiconductor layer 106 or into the oxide semiconductor layer 106.

The Vg-Id characteristics in FIG. 13A show that the threshold voltagesof the transistors included in Sample A3 at a drain voltage of 1 V isdifferent from those at a drain voltage of 10 V.

The Vg-Id characteristics in FIG. 14A show that the threshold voltagesof the transistors included in Sample A4 at a drain voltage of 1 V isdifferent from those at a drain voltage of 10 V. Furthermore, it isfound from FIG. 14A that some of the transistors do not have switchingcharacteristics.

The Vg-Id characteristics in FIG. 15A show that on-state current isreduced in the transistors included in Sample A5.

(Band Diagram)

Next, measurement was performed using a spectroscopic ellipsometer toobtain a difference between the energy Ec of the bottom of theconduction band and the energy Ev of the top of the valence band, thatis, the energy gap Eg of each of the following layers: the oxidesemiconductor layers and the metal oxide layers of Samples A1 and A5,the oxide semiconductor layer of Sample A2, and the stacked oxidesemiconductor layers of Samples A3 and A4. Furthermore, an energydifference between the vacuum level Evac and the valence band top Ev,i.e., the ionization potential Ip, was measured by ultravioletphotoelectron spectroscopy (UPS). Then, an energy difference between thevacuum level Evac and the bottom of the conduction band Ec, i.e., theelectron affinity χ, was calculated by calculating a difference betweenthe ionization potential Ip and the energy gap Eg, and a band diagram ofeach sample was obtained.

FIG. 11B shows a band diagram of Sample A1. FIG. 12B shows a banddiagram of Sample A2. FIG. 13B shows a band diagram of Sample A3. FIG.14B shows a band diagram of Sample A4. FIG. 15B shows a band diagram ofSample A5.

As shown in FIG. 11B, a difference in electron affinity χ between theoxide semiconductor layer (IGZO(1:1:1)) and the metal oxide layer(IGO(7:92)) is as large as 0.5 eV. Furthermore, as shown in FIG. 14B, adifference in electron affinity χ between the stacked oxidesemiconductor layers (IGZO(1:1:1) and IGZO(1:6:4)) is as large as 0.5eV.

Meanwhile, as shown in FIG. 13B, a difference in electron affinity χbetween the stacked oxide semiconductor layers (IGZO(1:1:1) andIGZO(1:3:6)) is as small as 0.2 eV in Sample A3. Furthermore, as shownin FIG. 15B, difference in electron affinity χ between the oxidesemiconductor layer (IGZO(1:1:1)) and the metal oxide layer (IGO(2:1))is as small as 0.2 eV in Sample A5.

These results indicate that, as shown in the case of Sample A1, whensuch a metal oxide layer as shown in Embodiment 1 is used as the metaloxide layer provided between the oxide semiconductor layer and the pairof electrode layers, a band offset of the bottom of the conduction bandEc can be formed between the oxide semiconductor layer and the metaloxide layer.

Meanwhile, as shown in the case of Sample A3, when an energy differencein the bottom of the conduction band Ec between the stacked oxidesemiconductor layers is small, a band offset of the bottom of theconduction band Ec is less likely to be formed between the oxidesemiconductor layer (IGZO(1:1:1)) and the oxide semiconductor layer(IGZO(1:6:4)), and thus carriers also flow in the oxide semiconductorlayer (IGZO(1:6:4)).

(Analysis on Cu Concentration by SIMS)

Then, diffusion of Cu in the metal oxide layer or the oxidesemiconductor layer in contact with the pair of electrode layers 110 aand 110 b in each of Sample A1 and Samples A3 to A5 was analyzed bymeasurement of Cu concentration.

Here, a stack including a metal oxide film and a copper film was formedon a substrate to form a sample. First of all, a process formanufacturing each samples is described.

(Sample A6)

Sample A6 was formed as follows. A 100-nm-thick In—Ga oxide film(IGO(7:93)) was deposited as a metal oxide film on a glass substrate.

Next, a 60-nm-thick copper film was deposited on the metal oxide film.After that, a 100-nm-thick silicon nitride film was deposited on thecopper film, and then heat treatment was performed at 350° C. for onehour in a mixed atmosphere containing nitrogen and oxygen.

Note that the metal oxide film (IGO(7:93)) was formed under the sameconditions as the metal oxide film (IGO(7:93)) of Sample A1.

Through the above process, Sample A6 was formed.

(Sample A7)

Sample A7 was formed under the same formation conditions as Sample A6 tohave the same structure as Sample A6 except that an oxide semiconductorfilm (IGZO(1:3:6)) was provided instead of the metal oxide film. Notethat the oxide semiconductor film (IGZO(1:3:6)) was formed under thesame conditions as the oxide semiconductor film (IGZO(1:3:6)) in SampleA3.

(Sample A8)

Sample A8 was formed under the same formation conditions as Sample A6 tohave the same structure as Sample A6 except that an oxide semiconductorfilm (IGZO(1:6:4)) was provided instead of the metal oxide film. Notethat the oxide semiconductor film (IGZO(1:6:4)) was formed under thesame conditions as the oxide semiconductor film (IGZO(1:6:4)) in SampleA4.

(Sample A9)

Sample A9 was formed under the same formation conditions as Sample A6 tohave the same structure as Sample A6 except that a metal oxide film(IGO(2:1)) in which the atomic ratio of indium to gallium is differentfrom that in the metal oxide layer included in Sample A6 is providedinstead of the metal oxide film. Note that the metal oxide film(IGO(2:1)) was deposited under the same conditions as the metal oxidefilm (IGO(2:1)) of Sample A5.

Next, the Cu concentration of each of Samples A6 to A9 was measured. TheCu concentration was measured using secondary ion mass spectrometry(SIMS). Note that the measurement of the Cu concentration was performedfrom the substrate side.

FIG. 11C shows analysis results of the Cu concentration of Sample A6.FIG. 13C shows analysis results of the Cu concentration of Sample A7.FIG. 14C shows analysis results of the Cu concentration of Sample A8.FIG. 15C shows analysis results of the Cu concentration of Sample A9.

Here, in a channel region of the transistor, the Cu concentration whichaffects the electrical characteristics is higher than or equal to 1×10¹⁸atoms/cm³.

As shown in FIG. 11C, a region having a Cu concentration of 1×10¹⁸atoms/cm³ in Sample A6 is a region which is closer to the substrate thanthe interface between the copper film and the metal oxide film(IGO(7:93)) by approximately 10 nm.

As shown in FIG. 13C, a region having a Cu concentration of 1×10¹⁸atoms/cm³ in Sample A7 is a region which is closer to the substrate thanthe interface between the copper film and the oxide semiconductor film(IGZO(1:3:6)) by approximately 10 nm.

Meanwhile, as shown in FIG. 14C, a region having a Cu concentration of1×10¹⁸ atoms/cm³ in Sample A8 is closer to the substrate than theinterface between the copper film and the oxide semiconductor film(IGZO(1:6:4)) by approximately 16 nm.

Furthermore, as shown in FIG. 15C, a region having a Cu concentration of1×10¹⁸ atoms/cm³ in Sample A9 is closer to the substrate than theinterface between the copper film and the metal oxide film (IGO(2:1)) byapproximately 15 nm.

Comparison between Sample A6 and Sample A9 reveals that the diffusionlength of copper (Cu) can be small in the metal oxide film in which theatomic ratio of Ga to In is high.

Comparison between Sample A7 and Sample A8 reveals that the diffusionlength of copper (Cu) can be small in the oxide semiconductor film inwhich the atomic ratio of Zn to Ga is high. This is because when theatomic ratio of Zn to Ga is high, the proportion of a spinel crystalstructure can be reduced.

According to the above results, the metal oxide layer which tends toform a band offset when it is in contact with the oxide semiconductorlayer, and is capable of reducing the diffusion length of copper (Cu) isprovided between the oxide semiconductor layer and the pair of electrodelayers, whereby a transistor with high on-state current and excellentVg-Id characteristics can be obtained.

Example 21

In this example, the crystal structure of a metal oxide film, the numberof particles during deposition of the metal oxide film, and the banddiagram of the metal oxide film were measured. The results aredescribed.

(Method for Forming Samples)

In this example, samples were each formed in such a manner that a100-nm-thick In—Ga oxide film was deposited as a metal oxide film on aquartz substrate.

Note that the samples were each formed using an oxide target having anatomic ratio of In:Ga=22:78, an oxide target having an atomic ratio ofIn:Ga=7:93, or an oxide target having an atomic ratio of In:Ga=2:98.Note that in the cases of using the oxide target having an atomic ratioof In:Ga=22:78 and the oxide target having an atomic ratio ofIn:Ga=7:93, a power (DC) of 200 kW was used. In the case of using theoxide target having an atomic ratio of In:Ga=2:98, a power (RF) of 400kW was used.

Furthermore, a deposition atmosphere condition where the flow rate ratioof argon to oxygen was 20 sccm:10 sccm, or a deposition atmospherecondition where the flow rate of oxygen was 30 sccm was used.

In addition, a substrate temperature was 200° C. or 300° C.

Note that in each condition of the samples, the pressure in a chamberwas 0.4 Pa.

(XRD Measurement)

Here, each sample was formed in such a manner that the substratetemperature was set to 300° C. and the metal oxide film was depositedusing the oxide target. Then, the crystal structure of the metal oxidefilm of each sample was measured by XRD. The XRD measurement results areshown in FIG. 16.

According to FIG. 16, the metal oxide films formed using the oxidetarget having an atomic ratio of In:Ga=22:78 and the oxide target havingan atomic ratio of In:Ga=7:93 have low crystallinity.

Meanwhile, a peak indicating a Ga₂O₃ crystal was observed in the metaloxide film which was formed using the oxide target having an atomicratio of In:Ga=2:98 under the deposition atmosphere condition where theflow rate of oxygen was 30 sccm. Thus, these results show that a Ga₂O₃crystal is included in the metal oxide film which is deposited in anoxygen atmosphere and in which the atomic ratio of Ga to In is high.

(Number of Particles in Deposition)

Next, analysis results of the relationship between the atomic ratio ofmetals contained in an oxide target and the number of generatedparticles are described.

A glass substrate was used instead of the quartz substrate in eachsample used for the measurement. Furthermore, the metal oxide film wasdeposited using the oxide target under the deposition atmosphereconditions where the flow rate ratio of argon to oxygen was 20 sccm:10sccm and the substrate temperature was 300° C.

Next, the number of particles on the glass substrate was measured beforeand after deposition of the metal oxide film with a test device using alaser. The results are shown in Table 1.

TABLE 1 IGO IGO IGO (In:Ga = 22:78) (In:Ga = 7:93) (In:Ga = 2:98)Particle before after before after before after diameter deposi- deposi-deposi- deposi- deposi- deposi- (μm) tion tion tion tion tion tion1.0-2.9 3 384 4 10 2 7 3.0-4.9 0 6 1 1 0 1 5.0- 0 0 1 2 0 0

Table 1 shows that the number of particles can be small after depositionby depositing the metal oxide film using the oxide target in whichy/(x+y) was greater than or equal to 0.9 where the atomic ratio of In toGa was represented as In:Ga=x:y. According to the results, a transistorcan be manufactured with high yield by depositing a metal oxide filmusing a target in which y/(x+y) is greater than or equal to 0.9 wherethe atomic ratio of In to Ga is In:Ga=x:y.

(Band Diagram)

Next, in a manner similar to that of Example 1, energy gap Eg,ionization potential Ip, and electron affinity χ were obtained using aspectroscopic ellipsometer and an ultraviolet photoelectronspectroscopy, and a band diagram of each metal oxide film was obtained.

Note that the metal oxide film in each sample used for the measurementwas deposited using the oxide target under the deposition atmospherecondition where the flow rate ratio of argon to oxygen was 20 sccm:10sccm at a substrate temperature of 300° C.

FIG. 17 shows each band diagram of a metal oxide film (IGO(1:1))deposited using an oxide target having an atomic ratio of In:Ga=1:1, ametal oxide film (IGO(22:78)) deposited using an oxide target having anatomic ratio of In:Ga=22:78, a metal oxide film (IGO(7:93)) depositedusing an oxide target having an atomic ratio of In:Ga=7:93, and a metaloxide film (IGO(2:98)) deposited using an oxide target having an atomicratio of In:Ga=2:98. Furthermore, as a reference example, a band diagramof an oxide semiconductor film (IGZO(1:1:1)) deposited using an oxidetarget having an atomic ratio of In:Ga:Zn=1:1:1 is shown.

As shown in FIG. 17, as the atomic ratio of Ga with respect to In in theoxide target becomes larger, a difference in the electron affinity χbetween the oxide semiconductor film and the metal oxide film isincreased.

According to these results, the metal oxide layer as described inEmbodiment 1, typically, a metal oxide layer in which y/(x+y) is greaterthan or equal to 0.75 and less than 1, preferably greater than or equalto 0.78 and less than 1, further preferably greater than or equal to0.80 and less than 1 where the atomic ratio of In to Ga is In:Ga=x:y isused as the metal oxide layer provided between the oxide semiconductorlayer and the pair of electrode layers, whereby a band offset of thebottom of the conduction band Ec can be formed between the oxidesemiconductor layer and the metal oxide layer.

This application is based on Japanese Patent Application serial no.2013-196333 filed with Japan Patent Office on Sep. 23, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An oxide layer comprising In and Ga, whereiny/(x+y) is greater than or equal to 0.75 and less than 1 where an atomicratio of In to Ga included in the oxide layer is In:Ga=x:y.
 2. The oxidelayer according to claim 1, wherein y/(x+y) is greater than or equal to0.78 and less than
 1. 3. The oxide layer according to claim 1, whereiny/(x+y) is greater than or equal to 0.80 and less than
 1. 4. The oxidelayer according to claim 1, wherein y/(x+y) is greater than or equal to0.90 and less than
 1. 5. The oxide layer according to claim 1, whereiny/(x+y) is greater than or equal to 0.93 and less than
 1. 6. The oxidelayer according to claim 1, wherein y/(x+y) is greater than or equal to0.75 and less than 0.96.
 7. The oxide layer according to claim 1,wherein y/(x+y) is greater than or equal to 0.75 and less than 0.95. 8.The oxide layer according to claim 1, wherein y/(x+y) is greater than orequal to 0.75 and less than 0.93.
 9. The oxide layer according to claim1, wherein y/(x+y) is greater than or equal to 0.90 and less than 0.96.10. The oxide layer according to claim 1, wherein the oxide layer doesnot comprise Zn.
 11. A semiconductor device comprising: an oxidesemiconductor layer comprising a channel formation region; an electrode;and an oxide layer comprising In and Ga, wherein y/(x+y) is greater thanor equal to 0.75 and less than 1 where an atomic ratio of In to Gaincluded in the oxide layer is In:Ga=x:y, wherein the oxide layer islocated between the oxide semiconductor layer and the electrode.
 12. Thesemiconductor device according to claim 11, wherein the electrode is oneof a source electrode and a drain electrode.
 13. The semiconductordevice according to claim 11, wherein y/(x+y) is greater than or equalto 0.90 and less than
 1. 14. The semiconductor device according to claim11, wherein y/(x+y) is greater than or equal to 0.75 and less than 0.96.15. The semiconductor device according to claim 11, wherein y/(x+y) isgreater than or equal to 0.90 and less than 0.96.
 16. The semiconductordevice according to claim 11, wherein the oxide layer does not compriseZn.
 17. A semiconductor device comprising: an oxide semiconductor layercomprising a channel formation region; an electrode; and an oxide layercomprising In and Ga, wherein y/(x+y) is greater than or equal to 0.75and less than 1 where an atomic ratio of In to Ga included in the oxidelayer is In:Ga=x:y, wherein the oxide layer is located between the oxidesemiconductor layer and the electrode, and wherein the electrodecomprise Cu.
 18. The semiconductor device according to claim 17, whereinthe electrode is one of a source electrode and a drain electrode. 19.The semiconductor device according to claim 17, wherein y/(x+y) isgreater than or equal to 0.90 and less than
 1. 20. The semiconductordevice according to claim 17, wherein y/(x+y) is greater than or equalto 0.75 and less than 0.96.
 21. The semiconductor device according toclaim 17, wherein y/(x+y) is greater than or equal to 0.90 and less than0.96.
 22. The semiconductor device according to claim 17, wherein theoxide layer does not comprise Zn.